Dont know about efficiency of code but it works fine for me.
Modify combination.tpl as:
{$sizes = array()}
{$i = 0}
{foreach $combinations as $k=>$v}
{if $v.quantity > 0 && $v.id_attribute_group == 1}
{$sizes[$i] = $v.attribute_name}
{$i = $i+1}
{/if}
{/foreach}
{$sizes = array_unique($sizes)}
{if $i > 0}
<div class="size-area">
<span>Sizes: </span>
{foreach $sizes as $a}
<p>{$a}</p>
{/foreach}
</div>
{/if}