MisterFubar
Rock Stacker
I suggest 3s and then just measure the area you want to put it in. Hobbypartz has the dimensions of all the batteries they sell listed. I'd just get as large as you can fit if you are just out having fun.
Remember that if you go 3s you need a BEC. With your 4WS you will be required to run at least one anyway though.
I imagine I'll notice a little bit more of a difference with the extra voltage, didn't notice much with the 2S I ran. Any brands from Hobbypartz to avoid?
I've already got the BEC covered, ordered it with the radio when I decided to steer the rear.