how large of a carb can perform well on stock 273 commando ?

ANY small fairly stock engine should have a vacuum secondary carb and not a DP or other mechanical secondary design. That's one reason the old venerable Holley "780" was so popular, the vacuum secondaries let you throw it onto about anything.

The AFBs and AVSs are also much more forgiving in that regard

I would think a 600 AFB would be about the top of the max