tire size for towing

i have an 88 chevy fullsize that i tow my toyota with, i need to know a good size tire for a 20 inch rim to tow with? also what backspacing and offset are the chevy guys running on these trucks to tuck?

Just try to make your self a good guess about the amount of weight you will be placing on the tires. You could even use a set of CAT Scales at a truck stop to findout the exact weight you are placing on them. Once you figure out that you can just look for tires that have a load rating of that or higher. But remember its just a rating, doesnt mean it wont handle more. The rating is just a safe guide to follow for proper wear and safety.