I have a 110 gallon tank, the dimensions are 48" x 18" x 31". I constructed the cabinet so that everything except for the RO/DI unit was self contained. The tank and cabinet are situated all the way against the wall.
First, we are a family of 6 - 4 kids and they come first.
Next my wife and I looked at room placement. Her criteria was simple, it could not render any room no longer suitable for its purpose. Easy enough. My criteria was simple, it had to be located on tile floor. The rest of our house is all laminate, so the kitchen/breakfast area was it. So basically, I only had one viable wall. This wall was 7-ft long, but I needed to still fit a garbage can, access the light switches and fit a cabinet that accommodated electrical, controller, RO/DI storage and salt water storage. Therefore I settled on a 6-ft cabinet with a 4-ft tank.
Regarding depth, we determined anything sticking out past the kitchen cabinets was excessive. We tried a mock up of a 24" deep tank and it was just too much. Sure, it is the ideal depth for a reef tank and I wish I could have gone otherwise every day I mess with my tank...but kids and room functionality came first. If they made a 21" deep tank, my name would have been all over it.
I looked at many options as I wanted the largest tank I could physically fit and keep the room usable for a family, and the 110 was it.
PS, don't mind the gate, we are house breaking a 12-week old golden retriever, it's not for the kids lol. Oh and yes, GHA is out of hand and I am tackling that now.