I have a 42

You can get your manual here:
http://www.retrevo.com/support/Dynex-DX-LCD42HD-TVs-manual/id/22974bh131/t/2/
This is for the DX-LCD42HD-09.

The ZOOM button on the top of the remote is the one you want to keep pressing until you get the picture size you want.
If you are using a HDMI cable for HD, the HD channels should fill in the screen automatically. The non-HD channels would have the empty spaces on the sides for 4:3 screen format.
Best to keep it on normal mode, so the top and the bottom don't get cut off.

UNDER MENU IT SHOULD HAVE A CONVERSION OPTION ,,USE IT