You'll want something similar to the King Arms 11.1v 1100mah LiPo. square sectioned rectangular battery.
Check airsoftparts.ca for specifics (sorry don't know ehobby's site very well).
Note...the link you referred to does list the max length (152mm)...but keep in mind that you're going to want a bit of space for wires/connectors in there too.
Also note that it says it comes with "T-Shape Type Connector Rear Wire" which I'm GUESSING they mean a DEANS connector. If they've done it right the male end is on the gun. Most stock tube lipos I've seen are mini Tamiya plugs...so you'll need to change the connector.