A 100amp service requires a minimum wire size of #1 AWG. Long cable runs would see an increase in wire size. Section 310-15 of the NEC is used to determine wire size, including referencing of table 9.
Depending on the length of the cable run, generally under 200'; the conductor size needed for a 100amp single phase service is #1 AWG. It's advised that a licensed electrician should be used for work on any electrical project. The National Electrical Code or NEC mandates minimum standards on electrical installations, in addition to both state and local ordinances. Another resource is the "Ugly's Electrical References, 2014 Edition" handbook, which can be purchased at most hardware and electrical wholesale stores.