Since I know Bren doesn't run lipo, I'll have a go at this one for him
The Nosram/LRP branded lipo stick pack (older ones are rated 2200mah, newer 2400mah) will JUST fit in the Xray radio box. You are advised to wrap the pack as in the pics below (click for bigger pics) to prevent damage to the wiring and also to protect the pack from rubbing damage. The most common area for this is where there's a plastic lump in the back of the battery box where the screw holds it to the chassis. I drop a small piece of foam in there to stop it punching a hole through the pack, but you will still need to replace the tape periodically when it wears.
The physical dimensions of the pack above are 85mm long x 17mm wide x 27mm tall - whatever you go for, if it's more than a mm bigger in any direction have a measure up before you buy!
Having said all that, there's been a couple of threads on here about li-fe packs which avoid many of the disadvantages of lipo (don't need a regulator, don't explode/catch fire if mistreated) and Hyperion do a pack that should be a perfect fit. They don't appear to last as long between charges as lipo, but you can charge them silly fast (check the C rating of the packs) so if you've got 2 you can easily swap them out as needed.
Don't try to compare mah ratings across different type packs - I run a 2200mah lipo and after 2x 20 minute finals, plus 4x 5 minute qualifiers, plus practice, plus messing about in the pits and warming the engine before a heat I can only put 950mah back in a pack at the end of the day!
Adrian