Take it to a bike shop that has a seatpost sizer.
Or they may have several old posts of different sizes they keep on hand for working on bikes.
__________________
1989 Schwinn Paramount OS
1980 Mclean/Silk Hope Sport Touring
1983 Bianchi pista
1976 Fuji Feather track
1979 raleigh track
"I've consulted my sources and I'm pretty sure your derailleur does not exist"