How big is the mITX market?
It depends for what segment.
Looking on Geizhals for simplicity:
86 ATX, 74 mATX, 3 E-ATX and 17 mITX.
It has to be said tho that when using motherboard manufactors sites. Like Asus, MSI, Gigabyte and Asrock. You end up with something like ~180 boards and ~10 mITX.
However if we sort by Z170.
57 ATX, 8mATX, 3 E-ATX, 8 mITX. (Note that those 8 mITX comes from 5 makers only)
Its is a very slowly growing segment. But the trend is clear towards smaller. However expect many years before it really changes in numbers.
But the mITX segment is a lot more muddy than that. Because half the segment is roughly based on ultra slim that kills of any dGPU hopes for example or other standard components. While the other half is more or less open for full size components of all kinds. Then you have a niche in between that fits something else.
It's a growing market that's for sure.
Hopefully it will grow faster. Was a nightmare to find a good mITX board this round.