Make sure it's going to be tall enough to allow you to walk around under it, but not so tall where you are loosing too much shade.
They come in different lengths and various hardware qualities as well. The sturdier the hardware the better. I like the square tubing myself. Fabric choice a factor as well. Sunbrella is tough to beat.
No matter how tall it is there is always a time when the sun will be at a position to hit you right in the face.
I bought a few yards of sunbrella to match the bimini. Cut it to size and the wife hemmed it nicely. I added a few grommets as necessary, got some matching parachute cable and a set of rail clips and viola....I got a vertical shade.
It uses the existing SS bimini rails on the top, and the deck cleats off the bottom. Put it across the bow at the windshield, or across the stern at the sun pad, or on either the port or stbd side. Folded up it fits in the glove compartment.
We can now eat dinner in the shade.