This seemed to happen out of the blue.
I have been using Opera for years.
I can't find anywhere in Gmail to change the default font that is used in Opera.
Ubuntu is Version 11
Opera is version 11.6
In Firefox the font is normal size.
I have tried playing around with both all the browser setting I can find plus Ubuntu system settings (I had seen some mentions of Opera using the system default font sometimes) but no success so far.
Really bumming as I have been using Opera for two years and if I can't resolve it I will not be able to use it.
I can switch to Firefox but I don't want to.
Best Answer
It looks like it's a bug in Opera 11.6+: http://my.opera.com/community/forums/topic.dml?id=1157352&t=1323469056&page=1#comment10836852
I'm zooming the web page when writing emails now (and zooming out when done). Works quite nice (and you get the editor ~fullscreen :)
[Note from michael: Accepted this as it "answers" the question, but also see workaround below!]