NOTE: Postal Plus.ini for later Windows releases.
Postal runs natively in 640x480 resolution and there is no option in a menu to change resolution. However, it's possible to enable upscaling in the Steam version to any resolution desired. Unfortunately, game only allows for 4:3 resolutions and it's going to use letterbox for widescreen resolutions. Developer is planning on making a patch to allow playing in widescreen resolutions.
--glframebufferobject --resolution 1024x768
It's possible to run either OS X or Linux versions of the game in Window mode. Also Steam version allows to run a game in Window Mode. To run it in Window mode, either press Alt+↵ Enter or add --windowed to launch parameters.