Demon Power do some small LiPo's, about the smallest I've seen.
http://demonpowerproducts.co.uk/index.php?cPath=46_142
The one I had until yesterday, when I sold it to a friend, was only 22.5mm thick and 45mm wide. It was a Demon DL5000 2s 5000mAh 25C which they dont make anymore, but most of them on there now are around the same size.
Hope it helps you fella.
Stu