Service entrance wire size required is # 2. That assumes the wire is a short run to the service panel.
Service entrance wire should be AWG #3 copper.
Use 250 size wire.
You need to look at the regulations that apply in your country. If in doubt, use a neutral wire of the same size as the live wire or wires.
For a 200amp service you need number 4 copper
As a service entrance wire you need AWG # 3/0 gauge copper.
Service entrance wire size is AWG # 3 copper.
The minimum size grounding conductor for a 100 amp service is a #8 bare copper wire.
A 100 amp residential service requires a size #8 copper wire, it should be insulated in green.
# 6 copper wire.