16" wheels (for a base model) with tall tires. I used heavier springs, but it's pretty much what you can get from coilovers. I cut and rolled a lot of the fender to put 31" tires on, you don't want to go there.
Pay attention to the driveshafts and rear support structure. It's a weak point and I stopped where I did to keep from stressing it more. On the Frog, Woody had
tig welded bracing added.
I cracked a sump plate going over a loose rock in Rallycross. I now have a piece of stainless sheet under the engine - not to support the weight of the car, but a good deflector to absorb much of an impact. My underbody plastics are reinforced with vinyl floor tiles riveted to them.
If you want to go with air, I have a compressor and manifold that I cut out of an '04 VW Touareg with air ride. If it may be useful I can send it to you for a low-low price.
Edit: Forgot to mention that both of the cars pictured have the corner radiators removed and a large radiator in the frunk. If you do increase the front tire size, make sure that it will not hit the radiator frames. The next time I build one of these I plan to keep the stock radiators while adjusting them as needed for clearance.