Every airline has limitations on the size, and in some cases the weight, of hand luggage. It's not simply a question of whether or not it will fit into the overhead locker. Many airlines are fairly relaxed about enforcing these limits, unless the plane is very full, but anyone who flies regularly knows that they cannot be relied on to always do this.

Ryanair is notorious for rigidly enforcing its terms and conditions - it's the price you pay for their very cheap fares.

However if there was misrepresentation when the tickets were booked (and if this can be proved) then that would be a different matter.

This is bad publicity for Ryanair but O'Leary never seems to be too bothered by that.