This might not be what you intended, but I found a fantastic, easy way to set font-sizes in CSS. We all (should) know that setting font-sizes in pixels or any other absolute unit is a bad idea. Relative units are the way to go.
If you get lost in understanding ems, percentages, etc. then you could always use this as a guideline (which should look consistent in all browsers):
The default point size for text in browsers is 16 points. If you style the HTML tag to 125% (20 points) and style its immediate child, BODY, to 50% (10 points), you ems become easier to control and understand throughout your stylesheet.
For example, by applying the rules below:
html { font-size: 125%; }
body { font-size: 50%; }
1em is equal to 10 points in all browsers. So if you want to set your H1 tag to 16 points and your P tag to 12 points, it’s as easy as dividing by 10, and then stick an em on the end.
For example,
h1 { font-size: 1.6em; }
p { font-size: 1.2em; }
This way, font-sizing in ems throughout your stylesheet becomes more manageable. If you then want to increase or decrease the font-size of your entire website, you just change your font-size in your body tag and the rest will size itself proportionally.
Gee, I hope that made sense.
I’m not a fan of using keywords such as small, xsmall, etc. From experience, different browsers treat them differently.
I’d stick to the using the stylesheet rather than relying on the editor to do your font-sizing. Define your styles, then reference them in the FCKEditor using the XML method. Hope it helps!