For smaller radius –the hydraulic press seems to be standard for ½ spheres.   yes, max=400mm.

For bigger radius:

I thought I could just order the pieces that would normally be welded together.. But I see that you may be using a different technique for larger sphere?

The center of this sphere is what I mean.  Even the rectangles would be ok.! Are these segments made with Hydraulic mold? (LEFT IMAGE)  

Or, is some kind of pressure from inside complete flat construction? Like a balloon? (RIGHT IMAGE) 

  yes, you are clever.  when right image  full with water, we use some kind of pressure from inside complete flat construction.    then we highly polished to mirrior surface. ( one TECH ,   A way )

I DID IMAGINE:

The mold has a very large radius, but it is only a section – small degree of a sphere.  ( another TECH,B way)   that is great ,david ,you can imagine two of my TECH making spheres.

Each pre-pressed plate gets welded together to make a whole sphere.          one whole piece of curved-plate, we must use B way.  

                                                                                                                                       stainless sheet  max:800X500,   can you understand?