There are bike shops that will measure you first. A good experienced fitter can often size up someone's body build, flexibility, etc., and be able to recommend brands and sizes that are more appropriate. So, they may start with some measurements or putting someone on a fit bike to help narrow the search right from the beginning.
The body is a greatly adaptable thing and will tolerate much for a short period, so what feels comfortable in a 20 min. test ride may be painful on a century.
You don't have to go this route to find the right bike, but it is an option.
Frends know gud humors when dey is hear it. ~ Da Crockydiles of ZZE.