Have a look at some of the diemnsions on this site
According to the website, EMG 81, 85, 60 and a couple others are 2.75"x1.50" (doesnt say what depth).
I guess you could look at all these mesaurements and make the routing as big as the biggest here, that way the guitar has a good chance of fitting many different pickups. Stewmac sell router templates, they must allow for a number of different brands, although i dont know the dimensions of the template itself.
In my opinion, i would buy the pickup first, you can then work from those dimension, maybe make the routing a bit bigger to allow for other makes of pickup you may want to install in the future. Thats what i did.
You mentioned the swimming pool routing method, that can affect tone (removing alot of wood) and ive also been told that its better to have separate pickup chambers so that theres something inbetween the pickups. This can obviously be ignored if you're only gonna use one humbucker. Can anyone else confirm the pros and cons of the swimming pool method?