Recent posts and first featured

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">.