I'm a major Colubrid fan...

The only milks that would need a 60 Gallon would be Hondurans, Blacks, and possibly a large Sinaloan.

Smaller Kings/Milks would be Mountain Kings and Mexican Milks. Pueblans don't get very large either. (Eastern Kings get pretty big, by the way)

For a 20 Gallon tank, I agree with a Western Hognose. But if you go with something else, the ones above would probably work. A Corn can stay in a 20 gallon for about 2 years.