Pete,

As I recall, I have a 12oz fuel tank in mine (I get 10-12 minute flights easily). Honestly I don't know if you could get a larger tank in there. You might be able to fit a larger tank if it were taller and filled up the space that exists under my tank. But, it might be difficult/impossible to install it in the plane through the hatch after the fuselage is built. I think if the tank were longer, you risk having it extend into the wing saddle area.

I built mine pretty muck stock. The only mods I made were to strengthen the landing gear blocks, use nylon bolts on the wings, and I added extra ply in the tail and a hardwood block in front of the wing so I could convert it to a tail dragger at a later date (did that this weekend).

It's a good plane. Easy to fly.