I am not surprised this doesn't work because the Alias is for note windows. Unfortunately I could not spot another selector for this. You can however set the default font
.AFDefaultFont:alias Specifies the default font for the skin. This style defines both the default font family (as specified by the AFDefaultFontFamily named style), the default font size, and default font weight. .AFDefaultFontFamily:alias Specifies the default font family list ("font-family" property) for the skin.
Note that setting the tooltip font to 20px does not enlarge the tooltip box and you probably end up with copped fonts. One option for you is to read up on skin debugging and to use Firebug in Firefox to identify a selector used with tooltips. For this, disable CSS compression in web.xml using the parameter explained here