This depends on what you're asking.
There are two ways you can do this:
1. You want to resize the portrait because it's too small/big compared to others... you'll need to use an external program to manually resize the portraits.
2. If you want to shrink/expand how ALL portraits in the screenpack look, open the system.def file in the folder containing your screenpack and find this line:
;--------- Portrait Scale and Offset -------
portrait.offset = 0,0
portrait.scale = .72,.72
The bold text shows how you would make the portraits appear smaller. Think of it as a percentage of the original size (.72/1 = 72% of the original size). Your portraits will look a little weird because MUGEN's resizing doesn't use any filtering, but it works every time.
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