6’6" here. I got a full “bike sizing” before even looking. That gave me a stack and reach (for frame and bars) to work with. That helped me narrow down what I was looking for…those numbers gave me confidence that I could get a matching fit from a few select bikes.
Ultimately I called around and found a shop with both a 64cm Diverge and 64cm Roubaix both in stock (the luxury of two options!) Granted I had to drive 180 miles to get there.