What you want is the largest diameter tires that will fit in the body  and the smallest diameter rims (Yes, yes, but "wheels" is sometimes confusing and I want to be clear that I am talking about the metal part, not the rubber part!)
Then a check for load carrying capacity.
My guess is that you'll find out that this results in a 15" diameter rim, and a small 75 series tire  and this will probably also result in much more load carrying capacity than you need.
Just for reference, current practice seems to be to total up the maximum load each tire will be carrying (worst case), then add 15%  then that is the MINIMUM load carrying capacity needed for the tire at the pressure selected.
