Mitsubishi's 42"-ers were probably the largest commercially available models, overhere in Europe they only came as commercial 'presentation'monitors, not as regular TV's.
Philips has had a 40 or was it 42 inch tube in the late eighties or so. But, never seen any pics let alone an actual monitor.
Widescreen model largest size probably were the Thomson (USA) 40" tubes. Believe these were also used in the Loewe TVs.
As for experimental models and technology demonstrators, could you post some pictures of that 51"-er?