I didn't really get any responses on more accurate dimensions, so I went with 20' even for the central bridge diameter. I used a bridge (deck 1) layout from the LCARS Blueprint DB as a start and scaled it to 1:20. That filled up most of my dining room table, so I rescaled to 1:32. That turns out to be manageable at about 22" x 33". I don't currently have a place to post the image, but I'll try to line something up (suggestions welcome.) Anyway, I need to mount the layout on a rigid base and then start the real work.