Building the Carry-On and Personal Item Size Checkers

Last summer I watched a gate agent at Stansted pull a woman's backpack out of her hand, drop it into a Ryanair sizer, and charge her €55 because it was half an inch tall in one dimension. The bag was "carry-on approved" according to the Amazon listing. She had measured it at home. The problem was that she had measured it against American Airlines limits, which is what every US-facing review compared against, and Ryanair is a different airline with different rules. I went home and started a sprea