What is an appropriate datatype for an RFID, and is there a standard definition for the format?
I don't like answering my own questions (feels like cheating) but here goes:

According to a post on StackOverflow there is no fixed datasize.  RFIDS can vary in both length and format, depending on the industry.

I will stick with varchar(20) for my own purposes, because the batch of RFIDS that we purchased all fall within that size limit.