> To clarify, I want to give horizontal games a larger shadow mask. Is there any way to > do this?
Yes. raster.ini located in MAME\ini\presets pertains to a lot of horizontal games if not all of them, so you'll need to figure out the exact values and slot them in there.
One way to do this is load any horizontal game you desire. Access the in-game menu with adjustable options with TAB/Slider Controls then make a manual note of exactly what you've changed and what that value is (You're doing this because changed values don't save when you exit MAME)
After that slot them into raster.ini (this is assuming you're running with -video d3d)
If you're using video bgfx then we need to do something completely different.