MSN Messenger Font size

I find that setting the font size to 9 or 11 in MSN Messenger works only temporarily, when I sign out and sign back in, the font size reverts to 10 or 12.

Is there some sort of an addon I could use to set the font size (messages) in pt or em permanently?

If you are use an open chat window to configure your font size then open the MSN Messenger window and go to: Tools>Options>Messages>Change Font.

See if that helps.