There is some info in the manual on how to change the screen resolution.
https://drive.google.com/file/d/0BwSDHIR7yDwSelpqM...
The easiest way is probably to right-click the exe file, create a shortcut.
The right-click the shortcut and go to properties and enter the command line.
(You can google for more detailed info on how to use command line parameters in Windows).
For example you can enter:
-screen-fullscreen 0 -screen-width 800 -screen-height 450
(Not all older machines support Unity trees, they are quite demanding on the graphics).