The battery boxes for the 1/16 Revo are 101.9mm x 35.5mm x 20.3mm but the actual usable space is only about 90mm long because of the way Traxxas designs Revo battery boxes to taper off at a certain point.
I use the Traxxas ones and they work and fit perfectly but if you want to cut costs, HobbyKing would work too.
I looked on there for you and I found the perfect battery that they make specifically for that truck!
http://www.hobbyking.com/hobbyking/s...6_models_.html