It’s a very individual thing on tank size. Need to consider not only your goals, but budget and time as well which is hard until own a few tanks, not that you haven’t of course.
I found personally over the decades I like nanos. Previously I had a custom 6’ that I found too much work for me so limited myself to 4’ but even then I decided to upgrade to a 15g IM. I say upgrade because for me it was!
I do think 40b is the sweet spot unless fish only. I see a lot of people follow, which I think is some of the worst advise in the hobby, to go big as you can. Many of these people have a terrible experience with huge tanks and leave the hobby. I wonder if we would retain more people if they had instead done a 3’ or 4’ tank. But again it’s personal.
I found personally over the decades I like nanos. Previously I had a custom 6’ that I found too much work for me so limited myself to 4’ but even then I decided to upgrade to a 15g IM. I say upgrade because for me it was!
I do think 40b is the sweet spot unless fish only. I see a lot of people follow, which I think is some of the worst advise in the hobby, to go big as you can. Many of these people have a terrible experience with huge tanks and leave the hobby. I wonder if we would retain more people if they had instead done a 3’ or 4’ tank. But again it’s personal.