Y-3 is so tricky to size on, especially when it's neoprene.
I got my first pair of Y3s the other day. Originally went for the Retros, but was surprised how much I liked the Qasas on foot. IMO, the Retros, though clean, were too 'normal' looking for Y3 and that price tag. The new SS16 Qasas I believe I can fit into my everyday.
I ended up going down a full size and a half because the rep suggested I go as snug as possible given the stretch that occurs with the neoprene over time. Now I'm regretting. Tried them on again at home and I noticed that when I take actual steps, my foot slides up and toes end up hitting the end (doesn't hit end when stagnant). Ugh. Love the design, just such a pain in sizing. By the way for reference, I wear a 9.5 (what I consider my true size) in UB and Nike, 9 in Yeezys, and got an 8 that is too small in Qasas. I think I'm better with a 8.5.