Welcome Justin. I hope I am justin time with my answer and that others will follow with more information. If your skillet has the size number on the bottom instead of a four digit catalog number the pan was made before 1924 when the catalog numbers began to be cast into the pan. The trademark with the large W common to Wagner and Ware started right around 1915. Diameter is measured anywhere you want to measure it. A 1924 Wagner catalog lists the diameter of the bottom of skillets and, in the case of a No.13, the dia. is listed as 12". For any style or variation of skillet Wagner probably made the pan for a number of years so pinpointing hte exact age is nearly impossible. A circa date is about as close as we can get. On the auction you link to it says that pan was made in 1924. Unless the seller knows when it was bought new and that the selling party had new stock he really doesn't know when the pan was made. That pan has no catalog number which should have been included on the pan by 1924 but not necessarily so.
Steve
