wats the minamum size of water boul that should be used?

it depends if your snake likes to have a bath or not i would put one in the the size of a small bowl, (for a baby corn/rat snake )
hope i can help
8)

The bowl needs to be big enough so that your snake can fit in he if he/she wants to but not too deep that it can't get out.

I agree with 00luke00 100%, but it is pretty hard to find something that your snake won't be able to get out of, I could put a sink in there and he'd be able to get out (and my rat snake is less than a year old). Just go with something big enough that he can comfortably fit into.