Aumentar e diminuir tamanho de imagens em <LI> de acordo com r

tenho uma <UL> com 6 <LI> e para ficar certinho um do lado do outro quero que elas aumentam de tamanho na tela maior e diminuam na tela menor, porem não sei como fazer, poderiam me ajuda?

HTML

<ul id="portfolio-grid" class="thumbnails row">
                        <li class="span4 mix visu">
                            <div class="thumbnail">
                                <img src="images/port/01.png" alt="project 1">
                                <a href="#" class="more show_hide" >
                                </a>
                                <div class="mask"></div>
                            </div>
                        </li>
                        <li class="span4 mix web">
                            <div class="thumbnail">
                                <img src="images/port/02.png" alt="project 2">
                                <a href="#single-project" class="show_hide more" rel="#slidingDiv1">
                                    <i class="icon-plus"></i>
                                </a>
                                <div class="mask"></div>
                            </div>
                        </li>
                        <li class="span4 mix pape">
                            <div class="thumbnail">
                                <img src="images/port/03.png" alt="project 3">
                                <a href="#single-project" class="more show_hide" rel="#slidingDiv2">
                                    <i class="icon-plus"></i>
                                </a>
                                <div class="mask"></div>
                            </div>
                        </li>
                        <li class="span4 mix comu">
                            <div class="thumbnail">
                                <img src="images/port/04.png" alt="project 3">
                                <a href="#single-project" class="more show_hide" rel="#slidingDiv2">
                                    <i class="icon-plus"></i>
                                </a>
                                <div class="mask"></div>
                            </div>
                        </li>
                        <li class="span4 mix digi">
                            <div class="thumbnail">
                                <img src="images/port/05.png" alt="project 3">
                                <a href="#single-project" class="more show_hide" rel="#slidingDiv2">
                                    <i class="icon-plus"></i>
                                </a>
                                <div class="mask"></div>
                            </div>
                        </li>
                        <li class="span4 mix digi">
                            <div class="thumbnail">
                                <img src="images/port/06.png" alt="project 3">
                                <a href="#single-project" class="more show_hide" rel="#slidingDiv2">
                                    <i class="icon-plus"></i>
                                </a>
                                <div class="mask"></div>
                            </div>
                        </li>
  
                        <li class="span4 mix visu">
                            <div class="thumbnail">
                                <img src="images/port/06.png" alt="project 1">
                                <a href="#" class="more show_hide" >
                                </a>
                                <div class="mask"></div>
                            </div>
                        </li>
                        <li class="span4 mix web">
                            <div class="thumbnail">
                                <img src="images/port/05.png" alt="project 2">
                                <a href="#single-project" class="show_hide more" rel="#slidingDiv1">
                                    <i class="icon-plus"></i>
                                </a>
                                <div class="mask"></div>
                            </div>
                        </li>
                        <li class="span4 mix pape">
                            <div class="thumbnail">
                                <img src="images/port/04.png" alt="project 3">
                                <a href="#single-project" class="more show_hide" rel="#slidingDiv2">
                                    <i class="icon-plus"></i>
                                </a>
                                <div class="mask"></div>
                            </div>
                        </li>
                        <li class="span4 mix comu">
                            <div class="thumbnail">
                                <img src="images/port/03.png" alt="project 3">
                                <a href="#single-project" class="more show_hide" rel="#slidingDiv2">
                                    <i class="icon-plus"></i>
                                </a>
                                <div class="mask"></div>
                            </div>
                        </li>
                        <li class="span4 mix digi">
                            <div class="thumbnail">
                                <img src="images/port/02.png" alt="project 3">
                                <a href="#single-project" class="more show_hide" rel="#slidingDiv2">
                                    <i class="icon-plus"></i>
                                </a>
                                <div class="mask"></div>
                            </div>
                        </li>
                        <li class="span4 mix digi">
                            <div class="thumbnail">
                                <img src="images/port/01.png" alt="project 3">
                                <a href="#single-project" class="more show_hide" rel="#slidingDiv2">
                                    <i class="icon-plus"></i>
                                </a>
                                <div class="mask"></div>
                            </div>
                        </li>
                        
</ul>

CSS

.thumbnail {    
     width: 225px;
    height: 225px;
    padding: 0;
    overflow: hidden;
    position: relative;
    border: 0;
    border-radius: 0;
    box-shadow: none;
    text-align: center;
    margin-right: -4px;
    margin-top: -4px;
}
.thumbnail .more {
    display:block;
    z-index:4;
    position:absolute;
    top:-100px;
    left:50%;
    margin:0 auto;
    ;
    width:80px;
    height:80px;
    font-size:40px;
    color:#fff;
    text-decoration:none;
    line-height: 70px;
    text-align: center;
}
.thumbnail:hover .more {
    top:40%;
    margin-top:-40px;
    -webkit-transition: all 0.3s ease-out;
    -moz-transition: all 0.3s ease-out;
    -o-transition: all 0.3 s ease-out;
    -ms-transition: all 0.3s ease-out;
    transition: all 0.3s ease-out;
}
.thumbnail h3, 
.thumbnail p {
    margin:0;
    padding:0;
    position:relative;
    z-index:4;
}
.thumbnail h3 {
    color: #FECE1A;
    margin-top:10px;
    font-size:20px;
}
.thumbnail > p {
    padding-bottom:20px;
}
.thumbnail .mask { 
    opacity: 0.85;
    filter: alpha(opacity=85);
    top: 100%;
    left: 0;
    width: 100%;
    height: 100%;
    background: rgba(24, 26, 28, 0.63);
    position: absolute;
    -webkit-transition: all 0.2s ease-out;
    -moz-transition: all 0.2s ease-out;
    -o-transition: all 0.2 s ease-out;
    -ms-transition: all 0.2s ease-out;
    transition: all 0.2s ease-out;
}
.thumbnail:hover .mask {
    top:0;
}

/*
3.2 Portfolio
***************************************/
 .single-project {
    margin-bottom:30px;
    background:#181A1C;
}
.single-project img {
    width:100%;
}
.project-description {
    padding-right:20px;
}
.project-description p {
    padding-left:0;
}
.project-title {
    margin-top:15px;
    margin-bottom:15px;
    border-bottom:1px solid #FECE1A;
}
.project-title h3 {
    float:left;
    margin-top:0;
    padding:0;
    width:80%;
}
.project-title .close {
    color: #FECE1A;
    margin-top:10px;
}
.project-info span, .project-title h3 {
    color:#FECE1A;
}
.project-info {
    font-weight:100;
    padding:10px 0;
}
.project-info span {
    width: 80px;
    display: inline-block;
}
.project-info div {
    margin-bottom:5px;
}
.close {
    float: right;
    width: 40px;
    height: 40px;
    text-align: center;
}
#portfolio-grid .mix {
    opacity: 0;
    display: none;
}