What size conduit and wires are needed for 250A service?
Hire an electrician. Do not install your main service yourself. It must meet code, it must follow many regulations to the letter, it must be inspected, and it must be done my a licenced professional. You will not save money by trying to do it yourself, and it is illegal. Chances are your power company will not connect service if it is not installed by a licenced professional.