It's going to be hard to beat the M11's size - they all appear to hit about the same there.
How important is sound quality to you? I just got the M6 and it does allow for offline use of Tidal and is incredibly portable, but the sound quality, well, it sounds like a $120 DAP. Another potential option is the M9 which is somewhere between the size of the M11 and M6 and, so I've heard, sounds pretty good.
Personally, I was not a fan of the M11. I actually returned mine. A lot of people liked it, I just wasn't a big fan of it. But, the size wasn't that bad actually - I had no problem tucking it into my pocket or backpack for commuting. I assume that would apply to DAPs similar in size.