You'll have to pull to measure - either the inside of the headtube or the outside of the inserted cup.
I've used plumbing Teflon tape to shim a slightly too small cup into a frame and so far, so good. I'm sure the beer can shim or aluminum tape shim (auto body repair tape) could work, too.
__________________
72 Frejus (for sale), Holdsworth Record (for sale), special CNC & Gitane Interclub / 74 Italvega NR (for sale) / c80 French / 82 Raleigh Intl MkII f&f (for sale)/ 83 Trek 620 (for sale)/ 84 Bruce Gordon Chinook (for sale)/ 85 Ron Cooper / 87 Centurion IM MV (for sale) / 03 Casati Dardo / 08 BF IRO / 09 Dogma FPX / 09 Giant TCX0 / 10 Vassago Fisticuff