You can take a regular humbucker mounting ring, trace around it at the location you want the pickup, and then draw in the "ears" large enough to accept whatever brand of pickup you're using but not so large they protrude from the mounting ring. If you're not using a mounting ring then you can do whatever you think looks best. There's no standard route. There are routes that will fit most or all pickups, but not a standard size route. If you're using Dimarzio's you might want the little triangle ears like some Ibanez guitars have. If you're using Duncans or Gibsons you might just want an oval, or even take a forstner bit and drill for the ears first, before routing the inner square. The "old standard" Ibanez route has one ear longer than the other to accomodate the old pickups that had 2 screws on one side and one screw on the other. If you're using that kind of pickup, or a Carvin with two screws on each side, you need a larger ear size.