Commercial kegs take less CO2 than homebrew because they're already carbed. If you force-carb your kegged beer, you'll only get somewhere around 5-10 kegs out of a 5lb. tank depending on how heavily you carb it. But, if you use priming sugar in the keg and just use CO2 to adjust the pressure and dispense, it'll last a lot longer.
Pint for pint, you'll use the same ammount of CO2 to dispense commercial as you will for carbonated homebrew.
1/4 "ponies" are 7.75 gallons and a 1/2 bbl is 15.5 gal. Corny kegs are 5 gallons. So if you can push 25 kegs of homebrew with a 5lb. tank, you can push 16 pony kegs of commercial with the same ammount.
But I agree, get the largest tank you have space for. I've got a 20lb. for general use and a 5lb. for portable needs.