This rather simple mod allows you to adjust the magazine sizes for the three vanilla magazines, and both shotgun shell types.
Magazines
Default magazine size: 30
Minimum magazine size: 5
Maximum magazine size: 100
Shotgun shells
Default size: 10
Minimum size: 2
Maximum size: 20
Attribution
Mod-thumbnail used from: https://www.flaticon.com/free-icons/ammunition, Ammunition icons created by manshagraphics - Flaticon