I've never worked in an open floor plan with more that like 100 people. But I seem to recall only being bothered by people within a few cube sections from me in any direction. I would imagine that at some point the rest of the people in the "room" are far enough away that it doesn't really matter.
The collaboration that an open floor plan is supposed to promote has for me more or less translated to 'i can clearly see you so that is an open invitation to come interrupt you'. For me it's mostly that plus the constant movement in your peripheral vision nearby and far away that makes it bad.
