I once saw a cape gopher snake that I remember because it was pretty. I wanted to buy it so I asked questions, and remember it was roughly 3.5 feet at 4 years. That sounded small, so I asked if that was normal, and the seller said it was a healthy animal. Who know if that was true. However, of all the cape gophers I've seen, I've never seen one I thought was over 5 feet.
Stay away from the pacific gopher snake. It's a great snake, but I personally owned one that was, while not measured, potentially as big as 7 feet, judged by the fact that it was almost as big as a boa constrictor who had been measured as 7'5"
I'd also recommend staying away from bullsnakes, because while I know that there are definitely some bulls that don't get big, there are few sellers who keep the kind of accurate records about their bullsnakes to really predict exactly how big they would be. If you didn't care about what kind of bullsnake you had, you could post an ad saying you would pay good money for adult bullsnakes of any kind, and the most important information you wanted about the animal was exactly how big and exactly how old. 20 people would respond with their unwanted normal bullsnakes they never thought would sell. If you didn't specify, the people who would lie about the size of their bull to sell it would assume you wanted a nice, big one, and exaggerate how big theirs was. They would be weeded out. Then you could see which were the smallest for their age and make sure they were small genetically, and not due to underfeeding, history of illness or some deformity.
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