Too small?!?
The 20 inch tires are really just a little larger than your stock tire size. They have a circumference that is three inches larger, which won't make much of a difference in the way the vehicle rides or handles (past any determent caused the smaller side wall.) The only problem you may run into is the tires being too wide; the 275 is the tread width. A tread that wide probably won't fit behind the fender skirt, and you may have problems is the tires rubbing. I would say try to find a set with a tread width 255 or below, and there shouldn't be a problem.