Hello MikeyG,
Many people will give you different answers I am sure...so take the answer I guess that resonates best within you.
The only thing I think people will agree on is that its a myth that keeping a snake in a cage too small will keep it small, you will just end up with a stressed and unhappy snake in a cage too small for her.
I have heard it said that if the snake, when stretched out, can stretch to one half of the tank, then she needs a bigger cage.
Personally, I like to give my snakes lots of space, and am happy I can do that. I like my snakes to be able to stretch completely horizontally, and not have to curl up or bend cause their cage is too small. Im sure people will say you dont need to give your snakes that much room, but I like it.
Sincerely,
Susan