From "mythfrontend" go to Setup->Appearance. The default for the height and width is "0" - this will cause MythTV to automatically size itself to full screen.
If the MythTV GUI width and height are not 0, mythfrontend uses these GUI dimensions and is anchored to the upper left corner of the X Desktop. If the GUI X and/or Y are not 0, the upper left corner is positioned at the specified coordinates. If the "Run the frontend in a window" box is checked, the window will have a frame and can then be dragged to any position on the desktop.
NOTE: When the GUI is full screen, you may see windows rapidly flipping on top of each other. If this happens you will need to set your window manager to ‘Click to Focus’ for windows to stack properly. The fonts for the GUI and OSD will scale to whatever sizes you use. Most font sizes can be changed in setup selections or in the .xml files under /usr/local/share/mythtv/ . Make sure to use fonts large enough to be read on a TV screen from a distance.
The full screen TV size is based on the X display size. For Xinerama, you can specify a screen in Setup->General. The TV picture will be stretched to fit the entire GUI area regardless of the capture resolutions used. However, during playback, the "W" key can to used to correct differences between 16:9 and 4:3.