Look at the weight of the battery recommended on the store, try to stay at or below that. I run 850mah max on my TT, some people do run bigger packs, it depends on where the finished plane balances and how fast you want it to fly. A heavier plane will need faster airspeed to generate enough lift.
I would build the plane first, then see how much weight applied where on the airframe gets it to balance. My battery sits 2/3rds of the way inside the fuselage so I can’t fit a bigger one in there, your build may end up different.