At what point do I start to worry?

[BCOrchidGuy]

Well, a 20 gallon can be a great tank, there is a lot of options for aquascaping, on the down side, not much of what you would use in a 20 would be adquate for use on a larger tank. A cube would be an exception, I started my 60 gallon cube by filling a 20 gallon tank, the lights 24 inch T5s that I had on the 20 are the right size for my 24x24x24 cube. I added a metal halide etc... A 90 is a nice size to start with and to most people it's not considered a large tank. Myself, if I were going to go back to a 90ish size I'd go for the 120 48x24x24, I think the 24inches front to back gives you a ton more room for aquascaping and building interesting structures. The nice thing about a smaller tank is, smaller pumps, smaller skimmer, smaller budget (with in reason). The nice thing with a larger tank is you have room to grow, it takes a lot more live rock to fill a 90+ gallon aquarium than it takes to fill a 20 gallon.
Whether or not you go to a 20 or a 200 gallon tank, I'd always recommend a sump, for the 20 gallon a 5 or 10 gallon aquarium would make a decent sump and a 20 gallon sump would be excellent. For a 120 gallon aquarium the 20 gallon sump would still work nicely. If you want a sump for a refugium, stick with the largest you can use, go with a return rate that isn't going to wash away your macro algae.
You've got lot's of options, look through the nano sections, look through the picture sections. People here have done the most amazing things with smaller tanks, and people here have done the most amazing things with big tanks and everything in between.
As for newbie mistakes, they can be expensive or you can learn from everyone elses mistakes and keep yours to a minimum. Always take your time with a new tank. Always Quaranteen fish and even corals, another use for a 20 gallon aquarium. Corals will survive just fine under a couple shop lights no problem, as Marie pointed out VHO can grow awesome corals, and believe it or not there used to be a fellow on here who grew SPS under Normal Fluorescents, they grew slower but they sure were pretty. Get your aquarium set up, fill it with tap water (if your tap water has phosphates use a phosphate remover), turn on heaters, pumps etc... make sure everything works and is where you want it. Let it run 24 hours and watch for leaks or other problems. Turn everything off, drain the water and then add RO or DI water, fill and turn it all on again, add your salt to a VERY high flow area and leave it 24 hours or more. Now you can add your rock, try different things, the main thing is leave room for water flow, and for fish to swim, an open design looks much nicer than everything just stacked like bricks. NOW you have to leave your tank alone, after a couple days measure for Ammonia, etc... Make sure the tank cycles properly, once the Nitrates are down and Ammonia is down then you can start adding coral and fish. Some people will tell you that you can add a few fish right away but if you do, you risk loosing them OR having to try to catch them if you put damsels in. Trying to catch them doesn't sound like a problem untill your at the point where every piece of rock you had in your aquarium is on the floor and the fish are still avoiding the net...
Guess I've been rambling again.

Doug

[LostMind]

my first tank was a 120g (on the second floor of an 80year old home). It was, imo an awesome tank. If I had kept it running longer (I tried to upgrade to a 320g and failed) it would have grown out amazingly I think.

I think I did so well because the tank size helped to buffer any mistakes I did...

[KrazyKuch]

I believe the biggest tank you can get away with, is a 230, mainly for the fact that it weighs about 3400 lbs, and apparently according to some framers I know you shouldn't excceede 300lbs/Sqrft....and the 230 sits at like 289lbs/sqrft.....But thats just been what I have been told! Not sure if it's true or not!