Is my new cage too big?

[Alexandra V]

..what?
If you go by that rule, you're saying the tank should be 1ft long and 6" wide. Which is fine, but then you recommend a 20 gallon which is closer to 2ft long.

I'm sorry, I meant to add in the "minimum" part. It's been a long day, my typing isn't that great. Thanks for catching that for me. The 20 gal would be closer to 2 ft long, but isn't huge for a snake of the size mentioned by the OP.