If you right click on your avatar name tag (or part of your avatar that is not an attachment), then select Appearance, then Hover Height, you will get a window. Via this window, you can set your hover height above the ground, via the slider, or by directly typing in numbers. This works whether you are standing, or sitting.
This functionality is separate from, and arguably replaces, the Hover setting in the avatar shape parameters.
Note: In viewer versions 4.7.3 and higher, Hover Height is also included in Quick Preferences.