55 x 40? x 20 is standardish with the second varying somewhat and yes, that's 21.5 in one direction and yes the largest 16:9 rectangle fitting in there has a diagonal of 24.6". But still, your problem is not only a gigantic sheet of glass but also thinness so your carry on can contain other stuff and a stand. The problem is manyfold: not only you need a great bag (I have a modified Red Oxx Sky Train where the enitre back is a flat monitor pocket, it doesn't get better than that) but you also need a great monitor potentially with a 3rd party power brick and stand.