MythTV is wonderful, and it's backend/frontend separation is ideal for my uses - as it means I can watch TV in my bedroom, where I don't have an aerial. However, it is quite obviously designed for people who use it as a complete Media Center, and not people who run it on their machine. I found the best thing to do is have a secondary launch icon, which loads MythTV with a different resolution.
The following instructions are for Ubuntu, but I did something very similar in Fedora.
First, copy the mythtv.desktop file.
sudo cp /usr/share/applications/mythtv.desktop /usr/share/applications/mythtvsmall.desktop
Then we'll edit this file with "sudo nano /usr/share/applications/mythtvsmall.desktop".
I then changed the "Name=" portion to read "MythTV Frontend (Small)", and the "Exec=" portion to read "mythfrontend -geometry 640x400".

640x400 is a nice size for my twin 1680x1050 screens, and keeps the widescreen aspect ratio. Moving the window around is as simple as holding down ALT and click-dragging on the window.
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