You COULD scale all the Eagle libraries down by a factor of 5, and then
lay out up to a 15x20 inch board, and adjust the gerber output in some
sort of gerber editor...  I ALMOST did that for a board that was big
but had very few components on it, but sanity prevailed...
It would be Wrong, though...  Even the full-scale version of Eagle is
pretty cheap compared to most PCB packages.

Rather than attaching multiple boards with wires, you might be able to
have them terminate at 'well defined' interfaces at the edges of the
individual parts, and then merge them with some sort of gerber tool...

BillW

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