Depends.
I'm on US9 on my 11s and 4s but for 6s, it felt abit big if I keep to US9.
But I realised it's the 6s having slightly higher toe space hence felt like too big.
Anyway, it's meant for casual wear so I don't mind even if it felt loose.
I went TTS.
Guys, alot of you mentioned going 0.5 up in size.
However, did anyone here have really break in the shoes while TTS and find that going up 0.5 is actually not necessary?