This festival was started in September of 1971 on a 22 acre field in Jonathan, MN. 25,000 people or even more visited the two weekend grand opening of the festival. renaissance festival mn (http://medievalfaires.com/minnesota-renaissance-festival-mn/) That celebration has since relocated to Shakopee, MN and found to be the largest Renaissance Festival in the United States.
Methinks you might find an argument on the size from TRF. Their 2012 attendance, if you include School Days, was over 600,000 for the season, with nearly 60,000 attending the closing day.
My friend Merlin the Elder is correct. I do remember seeing a total attendance figure for TRF at 606,000 for 2012.
An interesting FYI: The owner of TFR (Referred to as King George/I can't remember his last name) started the MN Ren Faire. He ran it for 3-4 years, sold the operation and moved to Texas. He then started TRF.
Mine's bigger ;D