From past experience, the liter limit for a carry-on is about 45L. If the airline/check-in agent is generous, you might be able to get a 50L or even 55L as a carry on but it's a toss up. Might not want to risk it if you're flying with a LCC that charges you for checked baggage. Keep in mind that if you do roll the dice with a bigger bag on a LCC you'll be paying more to check your bag at the counter if it doesn't qualify as a carry-on.
In the past I've gotten away with a bag a small as 28-32L in SE Asia where it's never cold. If you're going to a cooler climate, 40L will be pushing it unless you can pack super efficiently - 1-2 pairs of lightweight underwear and washing every (other) night, merino wool, stuff like that. Otherwise 50-55L is more realistic for cooler climates IF you don't have to bring boots.
As for which style packs, I've always had a top-load style pack, something like the
MEC Brio (older generation one). I haven't see the new ones but the old ones had a side zipper so don't have to dig everything out to get to the middle or bottom. The shitty thing about those are that there's no where to hide straps so if you're checking it you have to bag it to ensure your straps don't get snagged on the conveyor belts. The travel specific packs like the
MEC Supercontinental you can hide the straps which is convenient.
One thing you'll want to pick up for sure is at least one
stuff sack. This is gonna enable you to pack superlight. The flipside is that if you pack anything but synthetic material (ie cotton) you'll end up with crazy wrinkles in your clothes. Stick with polyester or a cotton-poly blend and you're golden.
One last note - have one of the MEC pack specialist help you get the proper size pack for you. Most of the MEC specific packs come in short, regular, or tall sizes depending on your torso length. The last thing you want is a "short" pack where the waist belt is sitting under your rib cage.
Good luck in your search and safe travels!