i use the protek 2100:
w w w amainhobbies.com/product_info.php/cPath/1389_1347/products_id/170974/n/ProTek-R-C-4S-Li-Poly-20C-Battery-Pack-148V-2100mAh-Starter-Box
it is super small and light, and has way more than enough power for a full rcpro weekend with 2 days of racing, and 1 full day of practice. say, more than a gallon.
these guys packing a 5000, or even 2, are hauling around a whole bunch of weight that they dont need to. maybe if your running 3 nitro classes, you might need 5000 mah.
my starter box weighs less than my car, by a good bit.