I think you're right for the MBA, I've always said that I'd love for it to be in three sizes, 12" for travel, 14" for everyday and 16" for desk-bound work.
I think that the Pro should be 14 & 16 or 15 & 17, I don't think they'd be able to make a pro-spec Apple laptop ultralight as well, and the 12" real estate would likely be too small for AV professionals.
Personally, I think that 18" would be too excessive and would start looking hideous, but who knows. I suppose the shell size would be the same as the discontinued 17" MBP, but looking back on pictures of that machine, it does indeed look hideous.