What size and type wire should I use to pigtail to my light switches?
Turn off the breaker for the device. Is it a 15-amp breaker? Use 14 gauge for pigtails.  Is it a 20-amp breaker? Use 12 gauge for pigtails.
Buy a few feet of the appropriate color of solid THHN and wirenuts. (THHN also comes in stranded, in this case you don’t want that.) As noted in a comment if you mean that you are going to use neutral as a bootleg ground, that’s a big no-no.
NM-B, or Non-Metallic sheathed cable is a multi-conductor wire with at least hot/neutral/ground, commonly called Romex.  THHN is a single conductor wire.