I will soon be building a stand for my 10g tank and I'm looking for a calculator of some sort that finds the amount of wood needed to build your project.
2 x 2 might work for a 10 gallon, but I would not trust 2 x 2 by itself, really, and certainly not for the larger tanks. As a frame that supports plywood or other sheet material, OK.
Much better to go with larger lumber for the larger tanks.
I use 2 x 4, and make open stands. Pretty much a cube of 2 x 4s, with center brace for anything 4' long+. A diagonal 2 x 4 creates the triangular stability.