For our upcoming trip, I feel like I just hit the jackpot with this tiny piece of luggage. We found it at Costco for $29.99 each. We bought four. When we got home, I measured them all around. I was surprised to see that despite the marked measurements of 16.5 x 12 x 9 inches, it was more precisely a 16 x 12 x 8 which is just under the maximum size for a personal item (AKA free item) on Spirit, Frontier, and American Airlines.
Since I couldn't believe our luck, I even took the little luggage to the airport to double check.
Here is a picture that compares the size of a regular carry-on size (you must pay for this one), and our new tiny luggage (again, it is free!).
I love that it fits a ton, has wheels, and lots of pockets. The top pocket is even spill friendly, and on the sides it includes shoe compartments. We have one for each one of us. I highly recommend them if, like us, you are a budget-minded traveler.