Maybe there's an easier way to do this, but try this:
Before the loop begins, maybe right after get_header(), paste this in:
<?php $i = 1; ?>
And then right before the endwhile;, on the line right above, paste this in:
<?php $i++; ?>
Next, right after the loop begins (i.e. after the_loop()), paste this in:
<?php if ( 1 == $i ) { $firstclass="firstpost"; } ?>
Now, find where your post's HTML begins. Probably something like <div class="post">. You want to give the first post an additional class, so that it says <div class="post firstpost"> or something. Assuming that it looks like <div class="post">, you would write
<div class="post<?php echo $firstpost; ?>">
Now you can just edit your CSS to modify div.firstpost however you want.