I'm happy with my current 90 gallon tank size. But of course I wouldn't mind getting a bigger tank. Only things stopping me is time, maintenance and finances.
Time is a big one. After coming from work I feel drained. Couple with the fact I have other priorities, I get to enjoy the tank like an hour a day on the weekdays and as much as I want on the weekends. Every two weeks I have to do 2 hour maintenance (20 gallon water change/cleaning). If I had a bigger tank, then I would be spending more time in terms of maintainance.
Of course finances is a big factor as well. If say I lost my job, would I be able to keep the tank afloat with a lower salary/temporary unemployment? Also I would have to spend more money upgrading such as more powerheads, more live rock, more sand etc. Depending on what I'm upgrading to like a 6 or 8 foot tank, that would mean new lights, new skimmer as well. If I were to upgrade to a 120 gallon 4 foot tank, it would be less drainage on my wallet as a lot of my current equipment is transferable and I'd only have to worry about more rock, sand, salt and water for water changes. That being said, where I live electricity costs are increasing rapidly so having a large reef tank is getting harder.
So currently I'm satisfied with my 90 gallon tank. Have a nice balance atm. But if I were to upgrade, I'd probably upgrade to a 120 gallon if there was a good deal on a tank and stand or maybe eventually a 180 gallon tank if money becomes less of a factor. These two standard tanks have the best reef dimensions imo.