I forgot to add that the other thing you're looking at when choosing a lipo is the "C" rating, and my advice would be to get 30c or better.. If you're not racing, don't waste your $$ on uber high c rated batteries though. You don't want to go too low, but there's no real reason to spend extra $ to get just a little bit more out of your battery. FYI, the "c" rating corresponds to the amount of current a battery can deliver both continuously, and then there's a "burst" rating.. similar to the rating that an esc will have, where the "burst" rate is something it can sustain for a few seconds, and the continuous rating is as it sounds. The number you're more concerned with is the continuous rating.
Like someone else said, we're not sure what chassis you're running this in, but for 1/10 scale chassis 30c is fine, maybe even 25c if it's a quality battery. My sons' 16th and 18th scale trucks both run 20c rated lipos and do just fine, but I've heard of problems with people trying to run that low of a c rating with a heavier truck (ie; more demand for amperage from the motor/esc)