there was originally 325 and 400 it was believed they changed
throat hit and wicked different juices differently, and they were pretty much standard..
then a few people started trying 500 and realized the TH was smoother and it seemed to wick anything just fine. soon word spread and it became the most widely used.
many venders still carry all 3 but it is accepted 500 will do the job fine.
some people have said 600 wicks even better but it is scarce and when it is available the vendors act like its a gift and charge out the yang for it