If you want it to float off of the ground, there's a Movement Height field in the Movement category of the unit in the object editor. Also, the Movement Type needs to be Flying or Hover for the Movement Height to take effect.
rtm2, he didn't say specifically that he wanted to change it in-game, but yes, that will also work, if that's what he wants, but only if the unit is has movement type Fly or Hover.