I'm building a few sets for my studio. What is the optimum radius of the curve, taking into account wall washing, or complete blackout? I'm painting the walls, and paint matching hard boards on the floor

A 2 foot radius is about the minimum with 3 and 4 feet being ideal IF you have the room.

I have a 4 ft radius  that I really love......  I work for several years with a 30 inch radius, and it was not that forgiving...

Just curious as to what you shoot or plan on shooting so much that you need a build a cyc.  I have owned or worked in many studios that had them and rarely needed them.  The  last studio, when we down sized, instead of building another cyc like we had in the old one, we just got a long and wide roll of vinyl, turned it over, painted it white and hung it from the ceiling.  Sort of a permanent seamless.  I know a lot of studios did the same thing. 

I can see if you shoot cars or large sets that may require something bigger like a wide cyc.  But if 12 feet wide will do...

My cove was gray.  I just added white paper to it when I wanted to go white.  It was 12 feet wide and I often wanted wider. I was shooting a lot of products at the time.  I think 15 feet would be idea for a general purpose people studio, although 20 would be nice.

The more you have the more you have to paint and light.