Looks maybe 18 mm ?
What is on the other side - maybe the head of a crowned king facing?
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https://hansonslive.hansonsauctioneers.co.uk/lot-details/index/catalog/153/lot/70365/English-Medieval-Jetton-Edward-II-1307-27-Type-1-Sterling-head-Fox-class-11-Copper-1-06g-19-78-mm-Crowned-facing-bust-pelleted-b
Description: English Medieval Jeton. Edward II, 1307-27. Type 1,
Sterling head (Fox class 11). Copper, 1.06g. 19.78 mm.
Obverse - Crowned facing bust, pelleted border.
Reverse - Cross moline, pellet in each quarter, pellet border.
Catalogue Ref: Berry, pl.2, no 4.