Reverb Room Size moduation

The problem is that changing the room size requires changing the delay line lengths, which creates a pitch shift if it is not instantaneous and causes the click you mention. A single change is possible by duplicating the reverb unit, writing to both delay lines, and fading between them, if one knows the new delay line length in advance--after which one needs to wait until the delay line is empty or flush the delay lines before changing again. Flushing delay lines in standard audio apps is a very compute -intensive operation. The best way to do it is to deallocate and reallocate the memory, which is typically only possible from C source code and itself poses other issues. Usually there is no way to know the new delay line length in advance. And reverb already being compute intensive, I am not aware of anyone who has done that. What audio production engineers do, if they have the resources for it, is put separate reverbs on separate sends on the mixing console for each audio source, which improves the diffusion quality, but typically they do not set them to different room sizes.