When you start composing a new message, the toolbar shows the font name and size chosen in preferences, but typing generates characters one size bigger than what they should be.
This is evidenced in two ways:
  1. Selecting text and increasing the size by one, does not change the selected text in any way, which means it was already that size.
  2. Selecting text and clicking on the current size shrinks the text size to the correct one, which means typing in a new/blank message was done using characters one size bigger than the supposed size.

This bug seems to have been reported before here: http://www.zimbra.com/forums/adminis...ing-email.html
No fix was provided.

The bug does not seem to be browser or OS specific. I have tested on IE, Firefox, Chrome, Opera (non-Ajax) on Windows and Firefox on Linux. Same behavior.
Also, the bug manifests for all users.

I have encountered this bug in ZCS 8.0.0_GAA, 8.0.2 and 8.0.3, and as seen in the link above, it manifests in 8.0.5 also.