Hi Dennis,
I went through a similar exercise a few years back at a former employer
when posed with the multiple setup/changeover dilemma (~32 different
board sizes, mixed technology, machine inserted/placed components, batch
processed, varying daily lot sizes).  The chosen solution was to
determine a standard panel size (sub-panel from fabricator's panel) for
all boards and maximize the PCB usage per panel.  Taking into account a
typical keepout area of .5" around the panel perimeter used by the
fabricator and other assembly necessitated factors, the resultant size of
the  4 sub-panels was 11.2" x 8.45" -- which correlates to 2 sub-panels
at 16.9" x 11.2" for your application.  4 different fabricators provided
panels with these dimensions and far as I know, production is still
running under this premise without any drawbacks.
One point of note is that the layer counts were 4 or less and that higher
layer counts may require more keep out area for the fabricator (thus the
smaller dimensions as quoted to you).
One other point, while I'm at it, is to determine what assembly
processing allowances are required dependent on your equipment,
transport/handling/ and fixturing that again will reduce the resultant
PCB size.  Good luck.

Steve

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