Welcome aboard.

At 18 months he's still got some growing in him, snakes continue to grow throughout their lives although they slow down as they get older. You're right a female will get bigger, if he is male he probably won't grow more than another foot (if that), a female checkered could easily reach over 3 feet. When it comes to space more is best, if you can fit a 4 foot viv then that's going to be room for a female and if you're right and he is male he's got plenty of room to explore.