I had my engine rebuilt with all forged internals so what would be the biggest safe size to run.

 

Talk to your engine builder, the max you can run will depend on how the engine was set up.

 

as Ray said, talk to your builder cause with out a parts list and how it was put together we'd just be guessing.
if you haven't used nitrous, or have very little time with it and i were you, i'd be asking your motor guy what would be a safe amount for YOU to run, not the max amount the motor will take.
with nitrous thats 2 very different things.