Sounds like lots of design questions to get answered before you get started... Lots of questions...
Re: wood. Plywood works well for us. Chipboard also works well for us (when you stain chipboard, it can really look nice).
Exterior grade plywood usually uses a nice waterproof glue, but, unless you seel it, it's still going to swell.
Strength of the wood, well, this is going to vary according to your design... If you're going to frame out the terrarium and attach a wood shell to it, you can get away with thinner wood. If not, you will need thicker wood. Beams will be lighter on your hands, but a bit heavier on your pocketbook... 
Waterproofing = sealing. Epoxy is probably your best bet for this. Raka makes a good marine-grade epoxy for sealing wooden boats. It's stinky, and nasty stuff to work with, but, it works well. Chris has also recommended Envirotex. Nicer stuff to work with, but a higher price tag. Your choice.
Staining. Personally, I like Lauan plywoods, as they are usually nicely finished and take a stain very well. Once you stain, then seal, you should be golden. We've tried sealing with just a cheap polyurethane, and we've tried just painting; both were disasters. Neither stood up to our reptiles' traffic patterns. Epoxy is the way to go for this, we've found...
Glass versus tempered glass versus plexiglass can only be answered by your own plans. What are you putting in this huge terrarium? Lizard or snake?
If lizard, avoid plexiglass... It scratches easily, and, while you can fill and buff the scratches, it'll just get scratched again quickly. For a powerful lizard (like an ig, or a large monitor), consider tempered glass. You can use thinner stuff that way, and still be comfortable knowing it will withstand tail-whipping and the iguanas' patented "leap and crash" maneuver... If tempered is out of your range, you'll need to use a thicker normal pane of glass to get the same strength.
For snakes, plexiglass is nice, if you secure it... If you don't properly secure the plexiglass, it flexes and bows and provides a nice escape route for these master Houdinis...
Hope this gets you off in a general direction of things to consider!
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Justin, Julie and the menagerie...
www.baskingwonders.com
