Hi to all!
I'm sorry to reopen this thread, but I'm new to Zimbra (I installed it for test purpose - we are looking for a way to replace Exchange) and I have the same problem.
Installed: Release 5.0.14_GA_2850.UBUNTU8 UBUNTU8 FOSS edition
Tested browsers: FF 3.0.4 (Ubuntu 8.04) - FF 2.0.0.20 (Win XP) - IE 7 (Win XP).
Testing on IE7 gives another issue - it was not possible to write the mail at all!

(but I suspect is a IE bug, so it doesn't really matter...

).
The problem is always the same: the editor doesn't change the font size to the chosen default setting. It changes only the font-family (Verdana, in my case), but not the size (always setted to 12pt).
I looked in bugzilla and I found the bug:
Bug 10526 – Allow user to set default font settings for HTML compose
But the status is FIXED (in version 4.5). Is there a bug assigned for version 5.0 that I can vote? Or is there a workaround for the issue?
Than you!
[UPDATE]
It worked for me when updating default font in admin panel!
