Receiver packs generally use the smaller cells. I believe they are just referred to as a cells. Like when you see the pre-built 5 cell hump packs, they're described as 2/3a cell packs. The 2/3 is the configuration, and the a is the cell size.
Actually, Nimh Rx packs are generally made from 2/3A size cells.
Dangit Matt, always beatin everybody to the punch. Although I need to correct.. 2/3A IS in fact the size of the cell.
Scroll down to the small Intellect cell. It is 2/3A size. That particular cell is a high output cell generally used to build packs for mini vehicles such as the RC18 series, Mini-T, M18 etc.