With PayPal Standard, on the confirm page, there a re a number of hidden form elements used to pass item information. A description is not passed, but the name is, you coul concatinate the size onto the name.
Change the name hidden form elemertn which will look like:
<input type="hidden" name="item_name_<?php echo $eCart1_Index+1; ?>" id="item_name_<?php echo $eCart1_Index+1; ?>" value="<?php echo $eCart1->DisplayInfo("Name"); ?>" />
to:
<input type="hidden" name="item_name_<?php echo $eCart1_Index+1; ?>" id="item_name_<?php echo $eCart1_Index+1; ?>" value="<?php echo $eCart1->DisplayInfo("Name"); ?>| Size: <?php echo $eCart1->DisplayInfo("Size"); ?>" />