div.image {
   width:<?php echo get_image_size( $size[1] ); ?>px;
}

<?php header("Content-type: text/css"); ?>

<link rel="stylesheet" type="text/css" href="cssfile.php?param=1" />

<style type="text/css">
  div.image {
   width:<?php echo get_image_size( $size[1] ); ?>px;
  }
</style>

<?php header("Content-type: text/css"); ?>

<div class="image">
  <img src="image.gif" />
</div>

div.image img{
   width: 100%
}