It seems I have the same issue...
Language of my user is Polish, browser IE7.
I tried "zmprov ma
user@testdomain.com zimbraPrefHtmlEditorDefaultFontSize 12pt" it did not work.
setting font to another size (10pt, using zmprov) partially resolved this issue - user now can write message, but when he changes font size (html ajax editor with signature) the font actually changes but the combo box in UI doesn't reflect the change
On different PCs on IE6, IE7, IE8 everything is ok...
Zimbra is zcs-5.0.16_GA_2921.UBUNTU8.20090429045342