FCKEditor A4 size

Hello,

I want reduce fckeditor to A4 size.
I only want the user to enter the exact amount of text that will fit in a a4 size page. Only width is important.

I have set

oFCKeditor.Width = '194mm';

but real measure is 176mm !
I have problem width conversion from pixel (for example table cell width) to mm.
Now I use following conversion

1px = 0.28mm

How I can express font size to mantain a4 width proportion ?
I'm using xsl-fo.

Thanks