Huh?Unfortunately 18s and 20s have the best load ratings for towing.
I'd just look for the highest load index or wieght rating you can find in your size.
I’ve looked at the specs on many brands/styles. The highest load rating for a specific ply, factoring in widths and height, goes to 16s, then 17s. 18s and 20s fall several hundred of pounds lower per tire. I had thought about going to a larger diameter rim on my tow rig (mainly due to availability, 16s aren’t stocked like they used to be) but all the published specs said I should stay with 16s. Is there something I missed?