@marialuisa.sacchi:
I, too, had this issue, as even my smallest roller board carry-ons are just a tad large. What I have found are very small, basic box-shaped wheeled ones by American Tourister, around $20-30 at Walmart and elsewhere. They are ugly, cheap and don't roll well, but they will fit.
I also found several duffel type ones that seem to fit almost exactly when fully stretched to the dimensions at Ross. TJ Maxx and Marshall's also have similar ones. They are also $20-30 and while they don't have wheels, they seem much sturdier. I purchased a Timberland brand one.
Finally, there are the under-seat luggage types. Benefits are that they have wheels and longer handles, so easy to move through the airport, but they cost around $50+ and take up much of the weight allowance. Good if you need more protection for items or need mobility assistance.
Oh, and my husband just flew the SFO-MXP and they were definitely weighing the carry-on bags.
Good luck and safe travels!