The easiest way would be using CSS, so you don’t have to change the PHP. Assuming you have a or some other wrapping element that contains all your posts, you could use something like
.main-content.span12 img {
width:50%;
height:auto;
}
.main-content.span12 h4 {
font-size:1em;
}
.main-content.span12:first-child img {
width:100%;
}
.main-content.span12:first-child h4 {
font-size:2em;
}
:first-child
will get only the first child of the parent element – so it will apply to only the first <div class="main-content span12">
.