to big of a tank?

[CntrlF8]

what happens when i use a tank thats bigger than the snake... like say the 4ft long tank with a 2.5ft long snake

In most cases they become stressed due to excessive space.

Just so you know, the larger tank you're talking about is a 50g. Most BP's don't need anything larger than a 30. Also length and width is much more important for ground-dwelling animals than height. That 50g tank is going to be harder to maintain heat and humidity, too. I say stick with what you've already got.