Late but wanted to chime in.

Although not possible right now, the best way to test chairs is to go to a Design Within Reach (a Herman Miller subsidiary). They employ chair specialists in the showroom who can work with you to find the best chair. People vary a lot and things like seat-pan sizes are not universal. Finding the right chair with the right options takes time.

I went in thinking I would get the Sayl and left with a Mirra 2 due its customizability which enabled full back support and a better seat-pan.

You can spend a whole day their testing the full offering. For instance, the Aeron comes in 3 sizes and without going and testing them you might not know which is best for you!