It's an age old topic. Roger Ditchfield recomended 3 -6 mm. After 6 mm he said you can suffer with instability problems so not to go any further.
I cut one from a 4 mm bit of plate I had lying around and I was very pleased with the result.
Just loosen the top nut on your shocker, lean the bike over onto the side stand, slip in the spacer, tighten the nut back up and away you go

As quick as that
Pete.l