Well, any pack smaller than 55 x 40 x 20 cms, really. That's Ryanair's maximum size and they are amongst the least generous. My Osprey Aura size M was allowed on board, but when we measured, my husband's male version in a size L would have been too tall by a couple of centimetres. They don't always check but why take the chance? Some popular packs on the camino, such as the Osprey Talon and Tempest 33/30 series, will go on no problem. Air gap back systems might present a problem if the pack is too long or too full, as the gap takes up a lot of room and the frame is not very flexible. Where are you living? That could determine what makes and models you have access to and what they cost. The main thing is that they carry 7-8 kgs comfortably for you! Good luck in your search!