I built a house with those exact same braces using 8' 4x4s. You can go longer but heed a couple things:
1) Pay attention to the directions they come with. Make sure you use 2x6, not 2x4s for the sides of your floor. You'll need them to put lag bolts into the sides.
2) DEFINITELY use cross braces on your legs. Run them from the very top of one leg to the very bottom of the other leg. This is where the support of your legs come into play.
3) The taller the house, the more careful you'll need to be when pulling it up to keep it from falling on over. Take your time and go SLOW. There's plenty of videos on youtube of folks pulling them on over on accident. The best way to built is modular. Build your floor first with legs and then pull it up. After it's up, add your walls and ceiling.
4) Use some type of anchor system. I drilled holes in my legs large enough to run rebar through them at opposite angles of the legs. I then bent the rebar at the top to act as an anchor.
