I did some measurements on screenshots of the marketing video, that gave me ~287x214mm.
Details
(I took the 30mm width of the expansion card as reference for the shot from the side where components are swapped out of the FW12, and then measured the aspect ration from the beginning of the clip where there is a pen writing on the screen.)
Comparing to the 297x229mm of the FW13, I think it’s going to be pretty difficult to fit in a FW13 mainboard with the same expansion cards. You’d have to somehow shave off 5mm on each side, and that’s going to be pretty hard considering that the width of the mainboard with plugged in expansion cards is almost exactly the width of the FW13.
Also, I’m pretty sure they would have at least mentioned the compatibility if it was actually there.
Then, of course, the next question would be why they didn’t “just” make the FW12 that 1 cm wider, but I assume the engineers and product folks thought about that too (and in much more depth than we can) and would have gone for compatibility if it was possible. Would still be interesting to hear the reasoning, tho.