The depth and virtual size are configured in the Section "Screen".

Depth stands for color depth or bit depth, is a computer graphics term describing the number of bits used to represent the color of a single pixel in a bitmapped image or video frame buffer. Higher color depth gives a broader range of distinct colors, but requires for larger frame buffer.

DefaultDepth integer
The legal value could be 8, 16, 24, 32.

When you specify your depth, X selects the bits per pixel (bpp) size in which to store the data. Below is a table of what bpp is used for each possible depth:

Depth	bpp
8	8
16	16
24	32
32	32

Virtual specifies the maximum of the virtual screen size. Without it the maximum virtual size will be limited to the size of the largest display that was connected when X was started. The maximum virtual size cannot be changed once X starts so needs to be large enough to accommodate the largest combination of displays you want to hotplug without having to restart X. Making the Virtual size square makes rotation easy. For example,

Section "Screen"
       Monitor  "Monitor"
       SubSection "Display"
              Virtual  2048 2048 
              Depth  24
       EndSubSection
       DefaultDepth 24
       Device       "Device[0]"
       Identifier   "Screen[0]"
EndSection

A bigger Virtual requires more memory. It's recommended to set video memory to 128MB in BIOS settings. Regarding to the maximum virtual screen size settings, refer to Part V. Chapter 6. Maximum Screen Size settings.