I don't think they even make 14" drag wheels anymore, do they?
15s are the smallest. You can fit a M/T 28x10.5W ET Drag out back, but it'll need adjustable suspension components and you'll have to BFH the wheelwells which is pretty standard for anything larger than a 275 tire in back. It's pretty standard to have to grind the calipers to fit most 15s too.There might be a couple options on the market now where this isn't a necessity.
If you're looking for the "biggest" drag wheel/tire you can fit back there without serious mods, I'm selling a set of 15x10" 7.5" backspace Pro-Stars with 28x10.5W ET Drags that are unused except for mockup and rolling the car around.