Hmm try looking at the openings and see which one it looks like. Also if yours doesn't have the "crack" on top like the xtra large does, then yours could be a medium or a large.

You can see the sizes here they show all pics of em: http://www.exo-terra.com/en/products/reptile_cave.php

Oh ya and also it may be 2-3 weeks before I get my bp because I'll be ordering another 16W mat (the one thats coming in will be used for my leopard gecko)