I'm not the definitive source on this, I'm definitely not management or even an employee, but in the past, about 256 pages seems to be a threshold for printing and binding. (A Word pages turns out to be close to a print page, which is handy) I haven't been the best at that - I think Spinward Extents, which went to 368 pages is the longest and that's after a bunch of things got repurposed to JTAS articles (there is still one starship that never made it... but I'll sneak it into something, eventually...). That seems to have added $10 to the hardback. This draft - almost done, but I think there are 5 -6 pages to squeeze in before the 101 vehicles... is at 157 before the outline of the vehicle list starts.... so it's close.
Me, I'd be fine with a 400 page book, but I'm guessing not-so-much as a reasonable product.
As for doing lots of TL16+ things... technically TL18 is in there but just barely. Not sure there is that much value for the top value (though the more I think about it, TL17 fabrication probably needs a paragraph or two in the vehicle (and things) manufacturing section).