I have a 26" licorice stick black rat in a 20 long, and a 24" black rat in a 29 gallon (same base size as a 20 long, just taller). They seem happy..however I do have climbing stuff in there for them also.

-----
The reason mainstream thought is thought of as a stream is because it's so shallow. -George Carlin
A fool doesn't learn. A smart man learns from his mistakes. A wise man learns from the mistakes of others. Which one are you?
My Website
Rat/Corn snake care sheet I wrote
Current snakes:
0.1 Licorice Stick Black Rat (Lola)
1.0 Neonate Black Rat (het for Lic Stk's) (Frankie)
1.1 Texas Bairds (Jose and Rosa)
0.1 Blue Beauty (Brunhilde)