This is what BA has to say
• Your main piece of hand baggage is up to the dimensions 56cm x 45cm x 25cm (22in x 18in x 10in), including the handle, pockets and wheels.
• Your bag must fit into the bag gauge, available at the check-in area.
• Musical instruments may be carried in addition to your stated hand baggage allowance, providing the dimensions do not exceed those of a guitar and providing it can be security screened
United says
For worldwide travel on any United flight, you may carry on one bag and one personal item such as a purse, briefcase or laptop computer. Your carry-on bag...
• must fit under your seat or in the overhead bin;
• must not be more than 9 x 14 x 22 inches (23 x 35 x 56 cm) or 45 linear inches* (114 linear cm); and
• may be required to travel as checked baggage if the bag cannot be safely stowed on your flight
Ryan’s site says
It should weigh no more than 10kg and not exceed the maximum dimensions of 55cm x 40cm x 20cm. Due to security restrictions - certain items cannot be carried in cabin baggage
so different airlines have different sizes and therefore one size DOES NOT fit all here ! That seems to be the root of the problem. The other issue is the a/c you’re flying. The bigger they get the more space you have. Is that right ?
so there is just no standard bag gauze that the hapless pax can use as a guide while choosing a carry on bag…