How do i get the right size serpentine belt?

I have a 400 amp alternator on the way and the pully on it is smaller than stock oem. so how do i go about getting the right size serpentine belt to fit proprly? I tried to have my mechanic help me but they are like "we dont want to be involved with aftermarket parts and install", Psshh. I hope i can just walk in and get it from napa or auto zone etc. So how do i figure out this belt size thing? The alternator manufacturer says the belt will need to be about 1/2" to1" shorter than stock