I have a long torso and long arms and at 70 I’m down to 5’8-1/4” from 5’8-3/4”.

I’ve been riding a 56cm for a long time. It is at least a size too big, but fore-aft balance, the saddle position to the BB, and how my arms stretch out to the bars without an abnormally long or short stem feels just right.

This may not be what you want to hear, but it really depends on the particular bike, not a generalized statement. Right or wrong, for me, everything is setup from the saddle position to the BB first. From there you can determine where the bars need to be. I probably won’t ride a bike so large I need an 80mm stem or one so small I need a 140.

Take angles and measurements from your existing bike and find one that will place you on the new bike. If you can fit on a smaller frame it probably will be a smarter way to go. I still have enough seatpost and a bit of straddle left, but one of these days I’ll probably need to move on. So far I haven’t spent enough time, but I’m not looking forward to it.

John