[FIT BMX]

A tank that big is going to take thicker glass, which is not cheap. Last summer I built a tank half the size you want, and it cost me $60.00 in glass, and silicone! You will need glass twice as thick as I used, for a tank that size.

On the up side, you will have a tank that it completely custom built for what you want. It is also a lot of fun building tanks to!!

 

[FIT BMX]

1/4" Would work good and 3/8" would be great! You could go thinner if you built a steel frame, the frame would take all the stress off the joints.

Building a tank is not to hard, it's even better if you have help. You will need a very very sturdy table, with a nice flat top!
Here is a used tank that is fairly close to you. It's really big, but you could modify it! AUCT Arboreal Lizard Vivarium - Vivarium Forums

Here is the tank I made.

 

[FIT BMX]

There's no such thing as to big!!
You could always chop it for the plexy glass.

One more thing on making a tank. Make some jigs to hold your glass in place, and use lots and lots of tape!