It will depend on the airline and you need to check that it is no bigger in any dimension than the size they give for any cabin bag. It will need to fit into the gauge at check-in for EasyJet for example or the infamous cardboard box for Ryanair. Be warned that many smaller but framed rucksacks that are sold as suitable for carry-on baggage are too long for Ryanair and because of the frame are not squishable enough for the box to slide over. So check the guidelines for the airlines you are likely to be travelling with and make sure you take a tape measure with you when you go shopping!

SM