If you are trying to add dampers to an Ender 3 you may either have the carriage ram into the stepper motor on the Y axis or you might have press fit stepper gears. Either way the solution is to print the adapter located here: https://www.thingiverse.com/thing:3245255
Instead of buying new M3 bolts, you can simply print these spacers and use 2 bolts from you X axis stepper. In general if you add a damper to your X axis, you will have 2 spare bolts, in some cases (if you use an adapter) you'll have 4 spare bolts.
These spacers let you use the longer 35mm bolts but be careful of the orientation. The top bolt must go in the rearmost hole or you will still have a carriage ramming issue.
Any but 0.3 to 0.4 is fine
If you have problems with bed adhesion, feel free to print a brim. I didn't need one but YMMV.
Also depending on your slicer, it might print faster to set it to "3 walls" because then instead of trying to do infill it just does a bunch of concentric circles.