All animations in SL assume a certain average avatar height. If you happen to be significantly taller or shorter than this average height, you might find that in certain poses, you sink into or float above the ground.

Fortunately, many SL viewers allow you to adjust your animation height via a height (or z-) offset.

In Firestorm, click the Quick Preferences Button (bottom-right on the bar): 
Click  then adjust the height offset.

In Phoenix, go to Edit > Preferences > Phoenix > Avatar and adjust the z-modifier.