I have my adult male Florida Kingsnake in a 48Wx24Dx18H inch front-opening vivarium right now. It depends on the snake, but mine is very active and uses every bit of that space; I am really glad I decided not to go with the smaller enclosure I was originally considering for him.
To me I'd think it would be a personal preference thing in regards to the front vs. top opening vivariums. I prefer front-opening enclosures for my own current snakes; they are easier for me to clean due to the space I am working in and my particular vivs are stackable which is a huge bonus.
