(Can you tell this topic gets me agitated?)

Good site regarding this issue:
http://www.thetravelinsider.info/tra...allowances.htm

General Luggage Policies

Note that when maximum size measurements are shown as a total number of inches (eg 45") this is the total of the length, width and height of the piece. There are standard sets of dimensions that go to make up these totals (for example, 22 x 14 x 9 is the standard for 45") and if you have a bag that is an unusual shape but still within the total number of inches, you may find it being rejected.

Is Your Carry-On Bag Legal?

Many luggage stores sell suitcases described as 'carry-on', but these suitcases are sometimes larger than the size most airlines will accept.

The safe maximum size is 45", in the form of a 22" x 14" x 9" bag. Some airlines allow up to as much as 55", but most do not.

Not only do luggage stores and manufacturers not always tell you if their bag is legally sized or not, but they also frequently mis-measure their bag. Their measurements generally are for the inside of the main compartment, and assume that any external pockets are of zero thickness, rather than stuffed full of things (which can easily add another inch or more) and ignore any external framing such as wheels and carry handle (which can also add another couple of inches).

If you should be very unlucky and find yourself forced to try and squeeze your carry-on into an unforgiving luggage template by the gate, even one extra inch - if your bag is already at the maximum - will be enough to mean it doesn't fit and you have to check the bag.