I am flying on Aer Lingus and Meridiana while in Europe and trying to plan for keeping within the baggage allowances. Meridiana states 55 cm x 40 cm x 20 cm (21.7" x 15.7 " x 7.9") while Aer Lingus sets limits at 22" x 18" x 10". I was thinking of getting the convertible carry-on from this website but at 9" x 21" x 14" - am I going to have trouble using this on Meridiana? How much do they pay attention to this measurement? Thanks.
The convertible carry-on shouldn't be a problem if you don't pack too heavy and really cinch tight the outside "cinch"' straps of the bag.
Your biggest concern should be carry on weight limits. Meridiana limits you to 8 kg (17.6 lbs) but Aer Lingus limits you to 6kg (13.2 pounds.)
I can not say for sure as it depends on the airline and the agent I think. I am sort of facing something similar myself.
Last time when we traveled Easyjet one one leg they did not make me check it then on another leg they made me check it as it was just over but the bag was a bit stuffed so it looked a bit bigger.
so I guess it depends on how stuffed it looks and the agent you deal with. Hit and miss probably