It depends on if you are carrying on or checking it in.  I’ve done both and I thinks it’s easier to carry on if possible.  You will of course have to do a little laundry along the way but with 3 weeks you can spare a couple of
hours to devote to doing laundry.  So the size of your pack will be determined by carry on standards which most airlines state as not exceeding 45” (length + width + height).   This translates roughly to a bag measuring 22 × 14 × 9. 

If you choose not to carry on, please do not go for the biggest bag you can carry.  I personally think travelers
using these look a bit ridiculous and can barely manage to move around sometimes.  It kind of takes away from
the easy, ready for anything, on the go kind of vibe that goes along with the whole backpacking thing.  Look here for
a few more tips on picking a pack.

If you are a light packer I would get a pack that fits carry on standards for sure and keep it easy.

My ideal pack would be somewhere between 50 & 65 liters, the upper end applying to a pack with a detachable daypack and including the volume of said daypack.

Thanks, i was thinking of a carry on only to save myself from watching the stupid bag carousel at the airport but i found a 60L backpack at a local outdoor store that is pretty good and was on sale for 70% off so i picked it up ($55 CDN), a bit bigger than i was looking for but for that price and the quality i couldn’t pass it up.

Thanks for the help tho

It depends on whether you want to pack the washing machine inside the pack or prefer to use a Laundrimat to wash your clothes.  If you know how to use a Laundrimat, about 40 litres is more than enough.  If you buy gifts you can always pick up a cheap sports bag on the way.
 
 

I took a 4 week trip 2 years ago and took a 72L pack.  I learned lots, and realized that half the stuff I brought I never needed or used.  I just had to lug it around for a month. 

I just returned from Europe on last Thursday, and I was gone for just about 2 weeks.  I brought at 36L pack this time and it was perfect.  Small enough to carry on (I didn’t cause I had a carry-on), or not.