"Why doesn't the desktop/MythTV interface fit properly on my TV screen?"
... is a question which crops up a lot.
The reason that the PC's TV output/VGA/DVI output doesn't fit _exactly_ on the screen is more than likely because of overscan. Most GUI elements _should_ fit the screen properly because the theme design guide specifies that a minimum of 5% of the screen's outer area be left empty.
A little bit of history
The actual display area of early TVs varied due to manufacturing tolerance problems. TV set makers needed to ensure that people didn't see 'fuzzy lines' or 'black bars' on the picture. The reason they might see parts of the picture they weren't intended to see was due to the aforementioned manufacturing tolerance problems. Displaying video signals to any degree of accuracy relies on rock-solid timing - something that just wasn't possible in the 1950s when TV sets first appeared on the market. By making TVs 'zoom in' to the picture to a small degree (sometimes as much as 10% or more), manufacturers effictively guaranteed every corner of screen would be filled with a watchable picture.
Sadly, overscan is still very much prevalent on modern TVs - even on some HD sets.
Making MythTV fit the screen on a TV
With Nvidia GF4 (or greater) VGA cards
If TV-out (composite or s-video) is being used, check out the nvidia-settings utility. This handy little program has a slider to control the amount of overscan applied to the TV output. A great deal of adjustment is possible..
It may even be possible to get almost all the desktop to show on TV without any black lines at the egdes. There is no guarantee that the screen can be resized or centred well enough to be able to see _all_ of the desktop .
To restore any nvidia-settings changes upon starting or restarting X, add the line
to .xinitrc or the X server startup script (or maybe even the desktop manager session file).
For everything else
If nvidia-settings did not solve the problem (or it cannot be used) help is still at hand. MythTV's GUI can be resized and repositioned from within the 'Screen Settings' section of the Appearance menu.
The GUI width (px) and GUI height (px) settings are 0 by default but can be changed by the PgUp and PgDn keys for coarse adjustment and the Left & Right arrow keys for fine adjustment. The default settings of 0 in the width & height fields make MythTV use the entire screen area, whose size will depend on the resolution X is currently running at.
A good starting point for the GUI size to make it fit a TV screen is 5% less than the resolution X is running at (e.g. 760x580 in the case of 800x600 screen res.).
If the GUI is off-centre, the 'GUI X offset' and 'GUI Y offset' fields can be used. Again, PgUp & PgDn keys give coarse adjustment while the Left & Right arrow keys provide fine adjustment.
Please note: changes made to the above settings do not take effect immediately. GUI elements need to be resized and the whole screen has to be redrawn before changes can be seen
A note about "Use GUI size for TV playback"
This setting will scale video playback to fit the same area the GUI occupies.
Broadcasters now take it for granted that TV sets will overscan the image they display (i.e. allow the image to spill 'off the edges'), so more often than not, strange artifacts (or even things like microphones) may be seen in the outer 5% of the image.
Adjusting the GUI width & height is almost essential for running MythTV in 'windowed' mode - with those settings the size of the 'window' can be determined.