Hi Katy, welcome to the forum. can you tell us a bit more about the bike? do you know what tubing it is made of? is the bike in the US or some other country? Pics?
check your local shop. lots of shops here have a set of guages, something like a Jeweler uses to check the size of a ring. if a shop near you has a set or a good pair of callipers it will be a snap to figure it out.
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“One morning you wake up, the girl is gone, the bikes are gone, all that's left behind is a pair of old tires and a tube of tubular glue, all squeezed out"
Sugar "Kane" Kowalczyk