Hey italian,
I've used this method with small cuttings and it works great, i'd say mine were no bigger than 3". As for them barely touching the water, that's just what you want, i have my stems around 1/4" from the water. i think, correct me if i'm wrong bozo, but i think bozo has his stems a little further away from the water than that, (the airstone makes the water spit up and onto the stem) so you shouldn't have any problems with them not being big enough, obviously you don't want to be taking crappy little 1" cuts, but my crappy little 2.5" cuts rooted just fine
Good luck!
e2a - Check out the first few pages of bozo's imaginary garden, it'll give you more of an idea how it works.