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

A Quadrajet is 750 cfm, Infinately adjustable secondary air valve, like an AVS or a TQ. Thermoquads are definately my favorite. I Dorked with Holleys for a while. They are fine for a race car, but an everyday driver at temps from -20 to 100 degrees, you can have them. I sold all of mine. I'm not a Racer, but I have friends who are. I the end, it comes down to tuning and keeping it in tune for the street. A factory AFB is tuned very well for a 273 Commando. There are 3 steps for enrichment, idle, midrange and WOT. It has an air valve tailored to the little Commando so it doesn't bog when you floor it, but opens when the 273 revs up. The OP asked about a stock Commando. I definately agree that you don't need to limit cfm by cubic inches and rpm. I proved that wrong a long time ago.