Typically we would say, "can I (or, May I) have a (small/medium/large) fry with that?"... or leave off the "can I/May I" altogether, just less-polite. Or "1 medium fry, 2 large" (implies you're still talking about the same thing, Fries. and we don't usually ask with "Don't you..", we would say, "Do you have this in a small (whatever size)?", or "is this the smallest size you have?" Hope that helps.