Sometimes the enforcement of carry on size is arbitrary and inconsistent. For example, I have a carry on that I have used for years. When I purchased it, it was labeled as consistent with carry on standards. My son and husband also have carry ons that are labeled as consistent with standards, and if you stand the bags up against each other, mine is actually smaller than theirs. This past March, when we traveled to Orlando, we took our usual carry on bags. On the trip to Orlando, no problem. Got right on the plane without a hassle. On the way home, however, the gate agent in Orlando made us check our carry ons because they were too big

. Since we had things like cameras and computers and ipods and ipads in our carry ons, we had to take them out, and, actually, once I removed my camera bag and purse, my carry on was empty! And both the camera bag and purse fit on the floor under the seat in front of me!
I can't imagine where I could actually buy a carry on that fits the specifications that the gate agent quoted. Perhaps if I could find a child's suitcase, that might work. But, in any case, after previous experiences of this type, I usually pack my carry on as if they are going to arbitrarily decide it is to big. I have a DSLR, and it's bag nearly completely fills the carry on I use. Last trip, I was able to stuff the Ipad inside the camera bag to take it onto the plane. But, there is no way I am going to check the camera bag.
And I further note that on this particular flight, there was actually loads of room in the overhead compartment, probably because they were making everybody check their carry on bags.