As Joe has said, those of us who wish to try LT (light truck) tires in lieu of ST (Special Trailer) tires that "Seem" to be much more failure prone (very costly if /when they blow out and rip up the side of your trailer) must move to 16" wheels in order to buy an LT tire having the needed load carrying capacity for the newer heavier trailers in the longer lengths.
It is nothing special about 16" per se - just what you need to do if you want LT tires depending on the trailer you tow!
Brian & Connie Mitchell
2005 Classic 30'
Hensley Arrow / Centramatics
2008 GMC Sierra SLT 2500HD,4x4,Crew Cab, Diesel, Leer cap.