If you have the ability to weigh your BP, you can go by that as a guide for ordering. About 10% or so of the body weight should do it. Most rat breeders size their offerings a little differently. Some show relative sizes on their web sites. Here's an example:
http://www.gourmetrodent.com/frozen.html
You said he had been eating Small to Med rats before you picked him up. I consider a Small rat to be in the 70 - 90 gram range. To be honest, that's about as large as I would ever feed my males on a 7 day schedule. Big females might require a Med rat, but with males, I tend to lean toward slightly under feeding instead of slightly overfeeding. Again, the ability to weigh your snake is important to keeping good growth and feeding records.