Chances are it is 25.4mm, or 26.0mm. Find a known 25.4 bar, if it doesn't tighten fully, the stem is 26.0 most likely.
__________________
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"