satakest

$wp_query->current_post

is the index of the current post within the loop so it may be easier to do something like this. (I’ used post_class() for the div classes, it is in WP 2.7 + )

//start the loop
<?php if (have_posts()) : while (have - blah blah blah ?>
...
<?php
$myDivClasses = array('unit');
// if it's the first(zeroth) post, size1of1 class else size1of2 class
$myDivClasses []= ($wp_query->current_post == 0) ? 'size1of1' : 'size1of2';

// add lastUnit class to odd divs (including first)
if ($wp_query->current_post % 2 == 0)
	$myDivClasses []= 'lastUnit';

// if this is the last post found it's a lastUnit
if ($wp_query->post_count == $wp_query->current_post + 1)
	$myDivClasses []= 'lastUnit';
?>
<!-- Now do your div with it's post classes -->
<div <?php post_class($myDivClasses); ?>>
...
</div>
<?php endwhile; endif; ?>

the []= operator adds a value to an array
current_post is zero based so add 1 to it for comparison