You're slightly misunderstanding. Or maybe I'm not clear enough. Probably both.
I get the menu to change the Screen Mode. It contains:
- Original
- Normal
- Zoom
- Full Screen
But in some games there is an extra item on the bottom:
- Custom
This is what I'd like to have in my eboots.
eboot I create look like this:
Hot Shots Golf 2 looks like this:
Edit:
Ah I figured it out!
These options come from the version of POPS that is being used to run the PS1 game eboot.
So by changing the version of POPS (by holding down R whilst game boots) to "original from flash", or maybe some of the later versions, then this Custom Screen Mode appears.
POPS versions 5.01 and 5.50, to name just two, don't have the Custom Screen Mode.