Revell made two Mayflower kits; it sounds like you have the big one. Both of them are scale models (very good ones, as a matter of fact) of the Mayflower II, the full-size reconstruction that was built in the 1950s and still lies at Plymouth.
The designer of the Mayflower II was William A. Baker, longtime professor of naval architecture at MIT. According to his book, The Mayflower and Other Colonial Vessels, the Mayflower II's keel is 58 feet long.
So measure the length of the keel on the kit - from the stern (not including the rudder) to the point near the bow where the extreme bottom of the hull starts to curve upward. (That's the bottom of the stem assembly.) Divide that dimension into 58 feet, and you've got the scale of the model.
Actually it's a little trickier than that - but not much. If I remember correctly, Revell molded the sternpost (the near-vertical timber to which the rudder is hinged) in port and starboard halves integrally with the rudder. In order to determine the length of the model's keel, you'll need to dry-fit the rudder/sternpost assembly to one of the hull halves. The sternpost (but not the rudder) needs to be included in the measurement of the keel length. (In reality the sternpost sits on top of the keel. So a fraction of an inch at the bottom of that "keel-sternpost assembly," as I've called it, is actually the aftermost segment of the keel. All this is about twenty times as hard to explain verbally as it is to see. Just get a clear idea in your head of how the styrene moldings relate to the wood components of the real ship; it's actually simple.)
My recollection is that the bigger of the two kits was a bit smaller than 1/60, but I could well be mistaken; I haven't seen it in a good many years. At any rate, it's a nice kit - perfectly capable of being a sound basis for a fine model.
Good luck.
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