With the seat post out, you can measure the OD of the seat tube ..
FWIW, tubes used in most bikes are fractional 1-1/8, 1-1/4, 1-3/8"..
the numbers, 28.6mm, 31.8mm, and 34.9mm are the metric equivalents.
then a manufacturer maybe, shims inside the seat tube ID
to use the common seat post sizes.