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

650 is a bit too much for a 273. Also, is it 4 speed? If not, get rid of the dbl pump.

Original D-Dart 273's had a 750 Holley, but they are a much more radical engine than street Commando's.

I like the orig carbs...Carter 4295 500cfm (thats whats on mine), or,if you want more modern, 500/600 Eddy.