I think it breaks down 2 ways. The right dimension for a tank not inwall with no fishroom, and the right dimension for a tank inwall (or sticking mostly or completely out of wall with the "hood" tied into the fishroom for venting).
Out of wall: I wouldn't want to go bigger than 120 or maybe 180. Its a complete pain larger than that. I had a 90 out of wall with everything under the stand it thing were a chore even with that size.
In wall the optimum tank is one that fits the space its built into AND is small enough that you don't mind doing maintenance on it. As mentioned above if its acrylic you can crawl around on top of it so with isn't an issue if you don't mind being spider man on top of the tank. Make SURE YOU WEAR A SHIRT! Mine gives my horribly acrylic burn
A tank when it gets this big is all about preference and your DIY skills. The space I wanted to fill and the size that I considered maintainable was 120x36x30. I'm taller so the 30" height isn't that much of an issue considering I'm going to have a few inches of sand, and I knew I'd be on top of the tank for any major work anyways. I picked 120" because thats the size that fit the space. I could have went another 2 feet but it started getting a bit unwieldy for access from the fishroom and I have a long and short viewable, and the shot side started getting a bit cramped. I went 36" deep because I personally don't want to maintain a tank deeper than that. Some people are ok with 4 or 6 feet, its just too much work for me.
One thing I did want to mention is on a mega tank with a fishroom IMHO you are nuts if you don't go with external overflows. It uncomplicates the inside of the tank. Mine come out another 8 inches so the tank is close to 4 feet wide dimension wise. With external overflows you dont have to make them the total height of your tank, they can be shorter. This greatly simplifies plumbing as you have more vertical space to work with and lets you have your sump out from under the tank, which leads to the next point:
If you have a fishroom, you'd be nuts to have the sump under the tank, if you have the room. Its so much easier to work on everything without being stooped over under a stand. Makes maintence more enjoyable.
I could go on and on, but my main point is, dimensions for a tank with and without a fishroom are two different things completely.