An important thing to remember in a tank the size you are talking about is access.
This bears repeating. Access, and also ergonomics. I don't like to have to stoop down, get on a ladder, or get my knee (or more of myself) into a viv. So having the top of the substrate about thigh or hip high, and the lights about eye height, and the depth no more than arm length, set the outer bounds on what I will build.
Then of course there's decorating - don't stick a bunch of branches or vines up near the front so you can't get behind them, etc. And I always preach bulkheads and sumps, and that's largely about access
to filter media, pumps etc.
There are numerous technical challenges and all of them will require an expensive solution in terms of time, money, or both.
I have found this true in the height dimension. Getting water way up, and light way down, can get expensive. My "outer bounds" gives a viv interior height of about 3 feet, which in all seriousness is plenty
. (You think you need more, get yourself a greenhouse OK? ha ha) I wouldn't go beyond about 2' deep either. But length? Hey, whatever you like, man. Just make sure you can get it in and out of the house. It will, some day, need to leave the house - rest assured of this.