They may be small, but mice are crafty. Let’s see who knows the science behind their sneaky skills.
About 1/4 inch — pencil width — is enough; if the head fits, the rest compresses, and young ones can do a hair less. Tried the flashlight-at-night trick to spot light leaks under doors and around utility lines? Fill anything that size with steel wool and caulk, especially the garage door sweep; CDC backs the 1/4-inch rule: https://www.cdc.gov/rodents/prevent-infestations/seal-up-holes.html.