Smallest snake for terrarium
 

Hello,

What are some small snakes, if any, that would fit in a 20gallon terrarium? It has water area and land area.

Storeria (red bellied or brown snakes) would be my first choice. They do well in planted vivariums. However they must be able to dry out, or they can develop skin problems. They shouldn't be kept in an overly wet and humid environment.
They also need an absolutely escape-proof enclosure.

Any species of Thamnophis would do well in an enclosure like that.

male house snake
 

A male house snake is about 2 feet long at maturity, small head, and very small in girth.

I would recommend legal Thamnophis (not sure what Jersey's laws are) and African house snakes. I don't recommend red-bellied or ring-necked snakes because they'll stay beneath the substrate most of the time and you'll basically have a tank with pretty plants and that's it. Garters have charming, curious personalities and accept a wide variety of foods. House snakes are a bit more subdued but tolerate handling readily.

When I kept Storeria, they actually spent quite a bit of time above the surface, basking and climbing in the plants.