There is no global setting because MAME doesn't support hstretch and vstretch in cfg\default.cfg, and there are no command line options for those settings.
You can make an override artwork file to scale the picture, but that would replace any existing artwork (like bezels and overlays), so I don't really recommend it.
Anyway, to do that, you'd make a file called percent.lay:
<mamelayout version="2">
<view name="95percent">
<bounds left="0" top="0" right="3000" bottom="4000"
<screen index="0">
<bounds left="75" top="100" right="2925" bottom="3900" />
</screen>
</view>
</mamelayout>
Put that in a zip called percent.zip in your artwork directory.
Include -override_artwork percent on your command line to test.
Add override_artwork percent to your vertical.ini to use.