I used 1/8" glass, but 3/16" will work, might be a little more expensive.
I've only seen it in 2x4' pieces, one should be plenty. I bought one about 2 years ago and use chunks for various projects and still have the majority of it left. Other option is to ask them if they have any that got broken in shipping that they would give you a deal on since you only need a small piece, or find some other local fishkeepers and see if you can get a smaller piece.
WOW cheech what a nice drawing! I think I will use it for my sump....very cool (sorry if I'm a few pages late...I'm out of town).
Be sure to post pics, Lady...I want to see how it turns out.
Robbie