Any that you like and that will fit in the head.
As far as what will fit, there are only two sizes--2.3 gallons and 5.6 gallons. The larger one is considerablly taller (full household height, in fact), but they both have the same footprint. Both sizes are available in portable (carry the tank off the boat and find a toilet or other suitable place to dump it), or permanently installed models that are fitted for pumpout. Like most things, portapotties come in all price ranges--from $25-$30 at Sears or Walmart to about $125 for a 5.6 gallon SeaLand that can be permanently installed (Model 965MSD). Thetford even makes an "electric" model that runs on D cells! And like most things, you get what you pay for. I'd go with the SeaLand 965 MSD if it were my boat...exceptionally durable and user friendly, and can be maintained and pumped out like any other holding tank. Check the archives (if Phil has finished restoring 'em) for previous conversations about "pumpable" portapotties. But whatever you do, don't even THINK of buying a recirculating toilet (a self-contained system that's "charged" with water and chemical and then just keeps rercirculating that AND waste until it's full).
