This question has many variables; temperatures of air and water, air circulation rate, etc. I'm not sure I could answer it even if I knew them all. Another way to go at it is to use information that you already have:
If your standing humidifiers work to your satisfaction, just measure the surface area of the internal evaporators. The required area of your water wall should be similar within a factor of two. A small fan circulating the air near the water wall will also be helpful.
LeeH
If your 'standing humidifiers' have some active element, extrapolating will be more complicated. Mike W.
(published on 03/24/2010)