That depends on the distribution-however it is generally part of the installer application.
When using centos you are brought to a command prompt when booting the install disk.
Here you can type in the following: linux resolution=640×480 to get that resolution.
The resolution= tells the installation program which video mode to run.
It accepts any standard resolution, such as 640×480, 800×600, 1024×768, and so on.