No, I wasn't necessarily talking about glass tanks, any type of enclosure really. I would say either 3'x3' or 4'x2' would do equally well for a 6' snake. But to get a 3'x3' enclosure, your either going to have to build it yourself, or pay quite a bit, whereas 4'x2' cages are extremely common and can be found quite cheaply.
When deciding how to house an animal, common sense goes a long way There are allot of people who have housed the same species in different ways quite successfully.
There really aren't any set rules compared to length for lizards I've heard, at least not a general rule. It really depends on species even moseso than snakes. If I had to make one up, I would say at least 3 times the snout-vent length (SVL) for the length of the cage, and at least half that in width. The best thing to do is to try to get some information from people who have successfully kept whichever species it is your interested in.
"Is bigger always better" is a question that stirs up allot of debate in the reptile community. IMHO, in general, yes. BUT, there are allot of things that have to be considered and possibly adjusted. If you just put a very small snake in a very large cage that isn't furnished and set up properly, you run the risk of the snake feeling insecure and at risk and never eating (this is a very common problem, especially with people bringing baby ball pythons and putting them in a 40 gallon tank.) Or a tine beardie in a large tank trying to catch crickets that are hiding all over the place.
There are allot of people who have housed the same species in different ways quite successfully. 
