I need to sell my husband's Cannondale Synapse. Can anyone tell me how I identify its frame size, please?
Usually, there is a decal/sticker on the frame in the bottom bracket area that will tell the size if it wasn't removed, typically in the range of 50 centimeters, or sizing like XS-XL. (Some brands also have it on the top tube or further up the seat tube, it depends).
Alternatively, you can go to a page featuring geometry data, look up the measurements for a bike and compare against the actual frame.
For example, here are the numbers for a 2022 Synapse: https://geometrygeeks.bike/bike/cannondale-synapse-2022/
You would need to know the approximate model year and model of the bike since there might be changes in geometry over the years.
I think, the safest measurement is the top tube or seat tube, these are often featured in small increments but it should usually get you to the correct ballpark of size if measured correctly.