The sizes are mainly fitted 7 5/8 (59Fifty), stretch or flex fit L/XL (39Thirty), snapbacks, and adjustable. There are several 7 1/2 and 7 3/4 sizes.
New Era NY Mets Blue Fitted Hat, Orange Bill Blue Stripe, Orange Logo, Sz 7 5/8
New York Mets Authentic Collection 59FIFTY Fitted | New Era
NY Mets Orange/Blue Stretch Fit ML Baseball Cap/Hat - New Era Caps
2016 New York Mets Alternate 2 hat
New Era Royal New York Mets City Arch 9FIFTY Snapback Hat
Men's New Era Royal/Orange New York Mets Authentic Collection on Field Low Profile 59FIFTY Fitted Hat
Men's New York Mets New Era Orange Alternate Logo 59FIFTY Fitted Hat
New York Mets Fanatics Branded Iconic Home Plate Adjustable Hat - Orange
New Era White New York Mets Pride Core Classic 9TWENTY Adjustable Hat
New York Mets Jersey For Babies, Youth, Women, or Men
Green New York Mets Orange Bottom 25th Anniversary Side Patch New Era 59FIFTY Fitted 7 3/8
New Era Brown/Mint New York Mets Walnut Mint 59FIFTY Fitted Hat
New York Mets New Era Cooperstown Collection 25th Anniversary Chrome 59FIFTY Fitted Hat - White/Orange