Most batteroes are made up from standard size cells, in our world, usually either AA or C size. 7.2 volts means 6 cells. The normal configuration is 2 rows of 3, giving a pack 5.5"X1.8"X0.9". If you find a pack with the cells stood on end, you get 1.75" tallX2.8"wideX0.9" deep. Two of these with a Y lead to connect them in parallel should take up the space described.
I had a look on C-S's website, but ran into a problem trying to see what was actually available in "large power packs".