Really making rules like this is silly. Like @Chowtime's tripod, we all have our "special snowflakes". My personal item is the smallest rolling duffel I could find (it's 17"x11" a bit smaller than the usual rolling laptop case at 12.5-13" so it should fit both United and Air Canada) but also surprisingly heavy since it contains my keyboard which is ~3kg alone (it's a vertical keyboard and the steel stand is heavy) and my spare battery which is 1kg alone plus some clothes plus the bag itself. It really gets heavy and that's why it's rolling -- but it fits easily underseat since it's not particularly big, all the above easily fits in a 17"x8"x3" space. It's just very dense.

And no, I am not checking it in because if I do and it gets delayed/lost how am I going to work on the other end when I land??