Baggie feeder shim size ?

depends somewhat on how full you fill the bag. I personally don't make shims up but use a dual top and bottom board with a 5/8" inch lip that works pretty well. I fill the 1 gallon size bag half full.

at other times I simple use an empty shell... usually a shallow depth box.

if I made up shims I would likely make them up like george imirie's shims (ie george imirie pink pages) which should be required reading for ALL beekeepers.