I believe all of these units can be used:
The most commonly used units are em, px, and % (same as em. 1em=100%, 1.5em=150%). In ebooks, using em is the recommended way.
Also, hopefully you are aware of CSS styling instead of inline.
More information can be found here:
Using CSS is a much preferred method to styling web pages/books because it provides an easy one stop shop for the entire formatting of your book.