2x4s, are really strong, as long as the design is good (aquarium weight directly supported by wood straight to the ground, screws aren't load-bearing, etc.)
I've also found the 2x3s fairly useful for some plans- and the big box places usually have those kiln-driedfor a bit under $2.
You could probably strip that design down to only 1 layer of wood on each leg, use 1x3s, and it would still be plenty strong. But, like etk300ex pointed out, you might run out of space for fasteners.
Not that there is anything wrong with overbuilding it, my 40B stand could hold a truck. But it was a pain to move outside for finishing and back in...
I kind of like using 2x4s and 2x3s, as you can get the kiln dried ones pretty cheap, and they make for fairly easy, really strong stands.