First of all, congratulations!
As far as collar size, I think you can make a pretty good guess-timate on size. I would look on some collar web sites because each vendor has a different way of doing it--some are one size only and completely adjustable.
Seamus weighs 80 pounds, and his neck circumference is 20 inches. He can comfortably wear a one-size, adjustable collar.
Edited to add: Is he in a foster home? Perhaps you can call the adoption director and ask if they can measure his neck.