I refurbed a couple of vintage hybrids for my parents a couple years ago. They are in their 60s and aren’t dedicated cyclists these days, although my mom was a bicycle commuter in her younger years.

Like you, hybrid sizing made no sense to me, so I guessed. My mom, who is 5’4”, got a 15” Trek Multitrack 720. My dad, who is 6’4”, got a 23” Univega Via Montega. Both turned out to be close enough.

I am 6’2” and ride 62-64cm vintage road bikes, but the 23” Univega is a little big for me. It has a 61cm top tube and a high BB, and I feel way higher off the road than on any other bike I’ve ridden.

On both, I swapped the stock flat bars to swept back cruiser bars for comfort and a more upright riding position. Worked a charm and was not expensive.

I think I paid $100 for the Trek and $130 for the Univega, of course this was in the Before Times.