European low-cost airlines
Ryanair: 40×20×25 cm free under-seat. Trolley 55×40×20 cm: paid (Priority or €55 at the airport).
easyJet: 45×36×20 cm free. Trolley reserved for Plus/Flexi.
Transavia: 40×30×20 cm free, trolley paid except 'Max'.
Vueling: same as easyJet overall.
Tip: a well-packed 35-40L backpack goes anywhere, a rigid XL trolley triggers extras.