You can see in the photos the size in relation to a person, but I think it helps to see the range of general size references for backpack volumes:

 > XS 7L to 14L - Kid size or purse size mini backpacks
 > Small "professional" flat form laptop backpacks 10-15 L
 > Mid-size day-packs 15-20 L
 > Larger backpacks 20-30 L
 > XL 30-45 L
 > Airplane Carry-On size 45 L (Cabin Max Metz Backpack)
 > 50 L and up - camping/hiking backpacks with frame support

The greatest thing about this backpack, aside from the nice clean box structure (as opposed to the bulging sports style of many larger capacity mountain climbing backpacks) is how incredibly light it is. At 1.7 lbs it is less than half the weight of the next closest competitor to the same size carry-on backpack (Amazon Basics is 3.6 lbs). I personally don't want an extra two pounds of zippers and compartments if I'm carrying it on my back, save the weight, extra compartments and stiffer structural elements for my roller carry on.

I would also note, that I filled the backpack with various towels and soft items so that I could measure it accurately, and in fact it is well under-filled when it measures the 21'' X 14'' X 9'' dimensions listed - if you are using it for some purpose that doesn't require fitting it into the airlines overhead compartment, you can probably easily stuff much more - I'd guess the actual capacity could go 50-55 L or more.