I wanna set font size in Pixels instead of "medium,large, sm

Hello everybody and sorry if it's a repost but I didn't find ant tips about this on the forum.

I would like to custom my FCK editor font size selector. Actually the system is with "small, medium, x-small" attributes, but when I compare
the page in firefox and IE it's not the same size at all. I tried to modify the page's code with size in pixel and it work perfectly.

So for resume, I want to be able to choose a size in pixel in font size selector instead of default size system.

I'm web designer and not coder at all, so it's very hard to find a way by my self, so if somebody can explain to me the full process for custom
the font size system as I want, I will be thankful

Hope that somebody can help, thanks !