The biggest we recommend for heavy towing is 190/30 or 205/30 and our KC stage 2.5... should be about 550-600hp on a good race tune and still tow max weight with ease as long as you are not running really big tires.
Like others have said a lot will depend on how you plan to tow. Are you just going to be going on a few short runs on flat ground at low elevation??? If so then you can get away with the 68mm and just get a good tow tune and watch your egts.
If you plan to go up to the mountains, tow at high elevaiton, or go up any really big hills then I would stay away from a larger 68mm turbo.
205/75 + our KC stage 3 68mm turbo should put you in the 600-650hp range on a good race tune, but it becomes hard to tow and we usually don't recommend it. Many tow just fine with no issues, just have to keep the rpms up.