Well, during my aisle shopping at the junkyards, I pulled two OEM Mazda wind deflectors.
One from a Millenia
One from a 626
The Millenia and the MX-3 Wind Deflectors are exactly similar. Both same width and height. Both mount points are similar.

The installed Wind Deflector is the Millenia, the one setting next to it is the MX-3.
The brackets supplied are a bit longer, but a pair of pliers and about 2 minutes on both solved that no problem.
The 626 Wind Deflector is a little bit longer and lower.

Black, front - 626; Green, back - Millenia.

Millenia

626

Millenia

626
I think the width is PERFECT for the MX-3.

It's exactly that position on the other side of the car:
I personally LOVE the look of the 626 Wind deflector. Some comparison shots:

626

Millenia
What do you think? This also means I have the OEM MX-3 Wind Deflector available if someone wants to purchase one.
I REALLY REALLY like the 626 one.

But... If the Wind Deflector is the same length as the MX-3 one, there's a good possibility the sunroof dimensions are the same.
This means I MUST return with a tape measure. The Millenia is glass, and this interests me greatly.