What is carry-on size?
If your bag measures more than 45" total (L+W+D), it is probably too big for the overhead bin
“Carry-on size” is determined by each airline individually, but is always measures by adding together all the dimensions (length plus width plus height).
Note that for many lately it has been slipping from the old 60" to around 45" total , so when shopping for a bag make sure it fits those smallest requirements. That way just about any airline will O.K. it.
Planes are getting more and more stringent about letting passengers on with massive duffels. United is even now charging extra for a carry-on in its lowest fare class.
Don't end up having your bag gate-checked when you really expected to have it for the long flight. Make sure you don't go over on the size limit. (Added bonus: Your fellow passengers won't glare at you as you try to wedge a steamer trunk into an overhead bin, hogging all available space and holding up the boarding process.)
Hint: With soft-sided bags, you can get away going over by a few inches, since you can squish them into acceptable-looking dimensions.