hi there, welcome back to the hobby
if youre going to run electric, you dont need receiver packs. the current radio systems run off the 6cell battery of the cars. i think some 1/12 racers use small receiver packs on their cars, but with the current high capacity cells being used, they are not mandatory.
if youre running nitro, 6v receiver packs mostly come in 2/3A and AAA sizes. 2/3A packs are a bit bigger but have higher capacities. AAA packs are a bit smaller but have a lesser capacity.
there are some lipoly packs that require specific chargers and converters/circuitry to run in nitro cars. but these packs are much lighter and smaller than any 6v nicad or nimh packs.
hope this helps