Mirroring two displays with different screen sizes
It is not possible. Apple explains well what happens when you mirror displays:

http://support.apple.com/kb/HT5019?viewlocale=en_US&amp;locale=en_US

"Your Mac will attempt to match the closest resolution between the primary (or built-in) and secondary displays. The usable resolution is the closest matching resolution for both primary and secondary display. If you choose a resolution that is not supported on both displays, the resolution will return to the closest matching resolution."

What are you doing that your only choice is to mirror the displays?

Cheers,

Dave