recommendation for carry-on size hiking backpack

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!

@terillp the rules around what size bag can be carried on vary amongst the airlines, and generally are a combination of maximum linear dimension (length + breadth + height) and maximum weight. Some airlines also set maximums for length, breadth and height rather than just requiring the total to be within the limit. The biggest pack that I have that fits the 115cm maximum linear dimension is an old Kathmandu Cane Toad at around 38 li, although on one trip the airline (Singapore Airlines) required me to remove some items to get it under their 7kg weight limit.

Once you know what airline(s) you are going to use, it is an easy matter to check their policy, but as you might have realised, somewhat more difficult to work out what packs might be acceptable.

Our Osprey Stratos (24 an 26L) fit just fine. And we really like them for walking a summer Camino (the hammock suspension was a godsend across the meseta).

I would guess anything between 30-35 liter size backpack. Last summer I carried on my 48 liter pack (American Airlines and Iberia), but it was far from filled to capacity. I cold have easily walked the Camino with a 30-35 liter size.

There are two issues here - what is the maximum size backpack that can be carried on as cabin baggage, and will that be big enough? The second is more difficult to tell without knowing the volume of the load that @terillp is going to carry. Without that information, suggesting a particular size pack will be suitable is really just a bit of a guess. A somewhat more informed guess is that for a summer camino, a pack size in litres about half of one's body weight in kg is going to be needed to carry a pack that is 10% of one's ideal walking body weight. There are still a lot of assumptions in this, but far fewer than saying 'I walked with a 33li pack, so everyone can.' It's just not particularly useful advice if the person is not of similar height/weight/build.