(On Win32) I'd like to have your experience before filling a bug. On some messages, not all of the size selections are working on the message pane directly. You have to refresh the message (by selecting an other one) to see it at 200% for example. It seems to be for signed messages where the signed part is changed: "This message is possibly signed and/or encrypted. This application does not currently support signed or encrypted messages." but not the body. (see previous thread from John Gardiner Myers to test) Am I the only one to see that?
