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 replaces, the Hover setting in the avatar shape parameters. Ensure the Hover setting is default 50.
Note: In viewer versions 4.7.3 and higher, Hover Height is also included in Quick Preferences.