The volume of CO2 produced Steve would depend on the volume of beer and how many gravity points of fermentation you get. I'd imagine you could use a kraeusening calculator to get a close guess.
I have some monster balloons we used to use for advertising big sales, until the price of helium went sky high, no pun intended. Fully inflated they were about 6 feet in diameter, and IIRC filling 3 of them consumed an entire 100 lb. bottle of helium - at $100 per refill. The point of this is that filling a balloon will generate some fairly substantial pressure, you might try a lawn and leaf garbage bag first, I think they are about 40 gallons. I attach a trash bag filled with CO2 to the airlock port now when bottling, just to minimize air entering the conical as it drains [laugh if you want].