There are very knowledgeable people on the forum who will disagree with me on this, but I think this is a bit of a personal preference issue. I prefer as small as I can use. I have what I think is an 18 inch (so old I cant remember), a 19 inch BRX and a rolling cabin bag (all B&R). I usually use the cabin bag and the 18 or 19 next. I also have a 22 inch and basically consider it a checked bag, although I have gotten it on planes as a carry on. The 18 fits in some very shallow bins perpendicular to the plane and that is VERY useful in terms of where you can find space for it. I have had an FA tell me to move it because it wouldnt fit in the bin on that side of the plane when they saw it was a roller and were shocked that the door could close.