Size depends on genetics,(ha-ha sorry),it depends on how many strings you want and the scale length.This link will help you calculate length:
http://www.buildyourownguitar.com/resource...c/jscrptclc.htm
The drill bit for the holes really depends on what tuning keys you're using.A lot of companies do either one or two piece necks.IMO-3 piece laminated is stronger,which is always best for a neck.