The thing is, I don't believe that it says anywhere that autosizing is a property restricted to the One Ring, or even the Great Rings. For all we know, it may be a feature of any magic ring made by the Mirdain. Tolkien never bothers telling us.
So it may well be that all Gandalf has to go on is "magic ring"- which tells him nothing new. And keep in mind that magic rings are not Gandalf's field; that's Saruman's speciality.
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The entire plot of The Lord of the Rings could be said to turn on what Sauron didn’t know, and when he didn’t know it.
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