KC,
I would go with a 4-5 foot cage. Keep in mind that colubrids need to be able to stretch out to avoid respiratory problems.
Also, a plastic dishpan with a hole cut in the bottom, placed upside down in the enclosure, makes a terrific hide/burrow. My two adult bull snakes love their "burrows" when not cruising the cage. I often walk into the herp room to see pit faces barely peeping out of the entrance holes.
My sonoran gopher, who is smaller, has a shoe box with a hole cut in the lid that he uses the same way--only difference is that I put some aspen substrate in the bottom of the box, and pile it up the sides to make him feel secure. It won't be long before he'll be big enough for his own dishpan/burrow.
Gopher snakes are fascinating animals. Best of luck with yours.
rgds,
althea