The only good answer is... experiment.

Since its rated for a 13X6, it should handle a 14X4 (Zinger and APC make that size.) It might handle a 14X6... but watch out for overheating (the biggest problem from too much prop.. the engine heats up as it tries to swing it.)

If you don't need to get the full power potential from the engine, and just want to swing a big stick... It'll probably turn a 16 to 18 incher, but you'd have to set the high end richer than normal, (low end doesn't change with the big prop.) and flame-outs durring aerobatics would be common.

I've put 12X5 props on .25's before... when I really only needed the power of a .15, but wanted a huge, slower turning prop. (powered gliders) You just have to realize the effects of what the big prop will be.