Font size

You cannot do that.

The font-size dropdown refers to the size attribute of a font tag. This is not specified in pixels, but an arbitrary scale.

None of my users have ever seen the font or font-size dropdowns ... and they never will. I disable them straight away as they encourage bad HTML (font tags were deprecated long ago).

A better solution is to create styles that reflect the appropriate combinations of font and size that have been designed for the site. After all, do your users REALLY need to display something in xx-large comic-sans?

See the documentation for info on modifying the fckstyles.xml file to give your users styles to select from.

An even better solution is to only allow users to format using font formats such as P, H1, H2, H3, H4 etc. You can then apply appropriate CSS to those elements.

Of course I'm talking about best practice content production for public web sites here ... if this is for internal use or something adhoc like a forum you might not care much about this stuff. If that's the case, let the comic-sans run free!!