You will need to do a load calculation. Find an NEC 2008 codebook. Article 220 has a load calcualtion you can use. I recommend you contact an electrician, who will be able to do this for you. You'll need to know what size of a service entrance you will need and this will be based on what appliances you will be using in the home. The use of central air, electric heat, and other appliances will be determining factors. You don't want to get this wrong and wind up with an inadequate electrical service!