What size conductor needed for 100 amp service panel?

In the US, with 120/240 volt single phase service the NEC standard is size 4-4-6 for copper and 2-2-4 for aluminum 3 conductor service cable (two phase wires and a neutral). BUT your local jurisdiction and/or power company may require larger cable or prohibit aluminum. You have to check with them on the size they allow and pull a permit to do this kind of work. Some municipalities will only allow a licensed electrician to take out a permit.