Since the sub-panel is a branch circuit panel board, and is assumed to be between the service disconnect and the main panel board. The National Electrical Code requires #4 copper wire for the 100 amp service.
You would need 4 gauge wire if you plan on running it up to 12 feet.12 gauge wire would work in a setup under 2 feet of wire.If you want to run the wire two to eleven feet then it would be a good idea to use eight gauge wire.
For 100 amp branch sub panel the size of the wire preferred is #4 gauge ution should be taken that the wire should be sufficient to be laid between the main panel board and the point of service disconnect. #4 copper wire or #2 aluminum wire can be used for this sub panel.