That's the size of a standard 28/32 qt. sweaterbox which is what most of the major breeders keep theirs in.
It's what I kept them in when I first got into snakes and felt it was a bit small. They'll do fine in there but it's your peace of mind that matters here.
Remember than the sides slope so you lose floor area. In the case of the typical 28 qt. sweater box that means a 20"x14" floor area. That's just a bit bigger than a 20"x10" 10 gallon aquarium.
You should ask this over on the cornsnake forum(s) as well.
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Current snakes:
0.2 Gonyosoma oxycephala - (Silver/Yellow)
3.3 Gonyosoma oxycephala - (Green)
2.1 Gonyosoma janseni - (Black)
3.3 Gonyosoma janseni - (Black & Tan)