Thanks for measuring that, very good information to have!
It's funny that this card is so long. The GALAX/KFA² 970 I've got is the shortest double fan non-LP GPU I know of, is advertised at 204mm length and because GALAX/KFA² actually measured the overall dimensions including the bracket flanges, it is actually 181mm long.
As others pointed out, the longest length ITX cards are 170mm long, everything above that isn't really mITX. I guess you could stretch it to a bit more as you need a bit of space for the ATX24pin connectors lock, too.
Interestingly enough, the price comparison site I use has a "special properties" field you can filter by which includes the entry "Mini-ITX Format (up to 185mm)". No idea where they got that number from.
The longest card I know of that was initially regarded as an ITX card was the Zotac 970 at 204mm, which I think we can all agree on, is not an ITX card by any stretch.
The minimum clearance would be 175mm to me. That way, truly-ITX cards will fit nicely, no problems at all. The maximum clearance is where it gets much more interesting.
To me, the maximum clearance is where the return rate of increasing the volume gets lower. It seems like that point is at exactly 183mm.
After now 1.5h of research and Excel magic, I came up with a bit of proof as well:
What you can see here is the number of cards you can additionally support per millimeter (aka the return rate) for every step my data could provide. I didn't have time to write a crawler that would scan every product page which would've given me data points for every millimeter, so I just used the steps the site provided. The data also isn't corrected for any wrong cards, so the EVGA cards are in there as 172mm, not 183mm and the GALAX 970 is in there as 201mm, not 181mm. EDIT: Data is now corrected for these units.
You can see that there are two main paths to go down.
You can either go very close to 170mm because going up to 180mm doesn't help too much.
Or you can go up to ~185mm and get quite a few extra cards, at least for nVidia. AMD only has the Nano and a few Sapphire cards in the game which are all 160-170mm.
The last graph is just all GPUs, no filters at all, so Qudros and FirePro cards and Matrox are included as well. It just strengthens the already established point that it makes no sense to go below 170mm at all.
After checking the cards in the 185mm range manually, I also saw that they were 183mm at maximum, so 185mm clearance seems like the a good number for the internal space for the card.
Why the fuck did I put so much time and effort into this?!
