There are some real small 2s hardcase lipo's intended for 1/18 scale cars. The capacity however is also pretty small, range 1800mAH.
LRP, CS Vampire IP are a few manufacturers that spring to mind.
Size is around
88 x 32 x 19mm (got 2 cs ones myself)
Don't think they'll be brca legal though and they will not be resilliant to a lot of amp draw!
On the bay you can still find some:
http://www.ebay.com/itm/LiPo-7-4V-18...item2ed3f77ca6