I first looked at tanks over 10 years ago when I got into the hobby. Originally thought I was going to do a 65 and ended up plunging into the hobby with a 120 and thought at the time it was huge.... Based on where the tank is located, anything bigger would feel a bit squeezed in - so no regrets on the 120 for my needs/constraints (just don't have the wall space for anything much longer). I am glad I went 24" wide (front to back) - as that extra room allowed me to have rockscape and room despite the interior overflow boxes. If I had to do it again, I would look at a tank with a single overflow- to simplify the plumbing and give me more room in the stand. Sump with refugium, reactors (carbon and GFO), skimmer, dosing containers, ATO reservoir, UV, and having dual drains and returns is a bit of a plumbing challenge. I also run full Apex (one of the items I invested in after the 1st year in the hobby) and am in the process of having an add-on cabinet built to match my stand so that I can move all of the controller gear out of the stand. One thing with this hobby - I think we grow into whatever size system we have - both stocking, filtration, and control. More volume is more stable to maintain, but the investment in the support system is also a bit more. As pointed out above - support elements that increase in quantity (lights, pumps) are the biggest cost items when adjusting system size, but for other items, going up a size in a skimmer, or up a size in return pump isn't as much of a delta (the larger sizes are an incremental increase, versus a multiplier by quantity). I don't regret starting with a 120 - it's a good balance.
