As a pool designer/builder I would probably install the 6'. The scale to the pool length looks better than a shorter one IMO. You may want to do a mock-up of the long wall with the waterfall in place. Some thin pieces of wood set up work fine. Then you can see it real-time.

The Sheer Descents requires 12 GPM per ft. So a 6' requires at least 72 GPM and a 4' requires 48 GPM.

We always use a separate pump and small cartridge filter when installing these, or any deck or laminar jets. We won't tie it into the filtration system.
Piping must be sized accordingly to move the water and accomplish the proper look.

Hopefully your PB has experience in sizing piping and pumps for applications such as this. The water fall should not be any higher than 3 1/2' above the water as the sheet will break up.

Thank you so much! Mystic Pools, LLC. Before reading your comment I made a mock up and actually loved the long design of the sheer descent. Your comment confirmed that is the best for this pool. Thank you.

You are welcome. Just be sure your PB sizes the pump and the piping for the proper result.

Thank you Mystic Pools. What height do you recommend? we were advised a 18''.

Set it at 18" above the finished coping which will be about 24" above the water.