Not sure about the old mdm but the new one just go into the html theme and change the theme.css file to make it any size you want for the text, not sure about the resolution part.
I'm fooling around with the new one but the old one should be in the same place usr/share/mdm/ part choose the one your using and just change the text size to what you want it should work.
all the mdm themes should be in that folder, and you can change what ever you want in them or make your own.
As far as the native resolution part, not sure about that one but at least you caan make the text bigger this way.
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