Wow, very cool stuff.
In my former life as a mechanical tech for Uncle Sam, we made extensive use of some sort of similar extrusions, but no where as nice as this stuff.
You could go fairly small and still get the strength you want. Especially if you design in proper supports. What I mean by that is - for like a box, put in some additional uprights so you don't have too long of a span that can flex. That sort of thing. Adding in panels will add a huge amount of rigidity also, so maybe play with a couple different designs and consider trade-offs in costs for things like extra bracing and associated brackets vs. what it takes to put in panels. Also, I didn't look at the specifics of the panels they provide, but I would imagine you could use just about any kind of panel material you want (don't have to go with strictly what they offer).
I really like the assortment of brackets, fixtures, and other widgets they supply to work with the stuff. I suggest spending some time (if you haven't already) browsing around all those to generate ideas on how to construct what you want.
I would also imagine that you should be able to get some specs on the stuff to get an idea of working load, how much a span of length X will deflect under load Y, etc.