Hoping I can shed a good amount of light on these NMD fitment wise.
So I wear 10 in UB. I could have went down .5 in NMD size to a 9.5 for sure.
BUT my wife's pair of NMD Key City just got delivered(thought it was my pair) and I put them bad boys on sz 8.5.
I CAN FIT THESE MFs...LOL. Now are they snug yes, but are they wearable YES. And that is WITH socks on. I tried to take some pics to see if they help anyone. I would say wearing a 8.5 is just a little more snug than how I wear my UB and I wear mine without going up .5 as a lot do.
So the verdict is you are able to go a full size down and still be good if you like snug fit. MAYBE 1.5 down but that might be pushing it. Without socks I might say you good. The material stretches that much.
Anyways...Here are some photos. My sz10 "lego" vs her sz8.5 "Key city"
Toes point up for reference