Building tanks

[Oliverian]

I posted this on AllThingsAquatic as well, but I thought I'd better ask on here as well, as someone here might know what I need to know.

I'm thinking of building some fish tanks (for fish) with a LOT of extra glass I have available for fairly cheap. I just need to know a couple things.

First of all, can this be done safely without having the tank suddenly burst at the seams one day and leak everywhere?

Is there a limit to how large I can make it? Like, I was thinking if I made too large of a tank, it may not be able to support the water pressure.

And what kind of silicon (or what kind of 'anything') is best to use for holding a tank together. It has to be non-toxic to fish, hold well, and not be too pricey.

And one more thing.. do you think it would be worth it? Or would it be cheaper to just buy a tank? I was thinking of making a fairly large one (IF it's possible). I just want to know if anyone's done this before, and if it's something that's worth doing.

Thanks guys. If anyone has any advice on this, please let me know.

-TammyR