We've carried on our bags many times and never had a problem with the size of the scanners. We were stopped once when embarking in Barcelona, and were told outside that we had to leave our large bags with the porters. I think they would have fit OK, but the policy there was to stop anything above airline sized carry-ons.
Our largest bag is considered a medium-sized checked back, that's 27x19x10. There are large checked bags that I agree would not fit through the scanners.