The values for DisplaySize can be calculated with the formula 25.4 * width / dpi or respectively 25.4 * height / dpi, 25.4mm being 1 inch. E.g. to use a dpi of 75 with a resolution of 1600x1200, the calculation would be 25.4 * 1600 / 75 = 542 and 25.4 * 1200 / 75 = 406, resulting in the setting DisplaySize 542 406.
Another way to get the values is to ask X server to calculate it for you. Run it using "startx -- -dpi 96" command, where 96 is required value for dpi and check the dimension:
xdpyinfo | grep dimension
dimensions: 1400x1050 pixels (XXXxYYY millimeters)
Use these XXX and YYY values for DisplaySize parameter.
i810 notebook users(samsung x20): Use 855resolution to get 1400x1050, most probably X won't start or the driver will use the next lowest resolution if you're just setting the xorg.conf to 1400x1050 without using the tool. Which looks like the font is blurred on every second line while reading this at tiny font sizes, couldn't see a difference @16+. Please add if this is also true for chips on desktop motherboards.
IF you are using nvidia's Drivers, a better way to do this would be to add this in your device section. This works alot better with dual monitors also:
File: /etc/X11/xorg.conf
....
Option "UseEdidDpi" "FALSE"
Option "DPI" "96 x 96"