I've been searching through the books for the answer to this question too and I'm sure it is an accidental ommission. In the magic section, the Bag of Holding has a volume and weight capacity so I don't see why the PHB doesn't have the stats for mundane containers.
Anyway, in "real rucksack world" capacity is in litres. 1 cubic foot is roughly the cube of 31 cm, or 0.03 cubic metres. There are 1000 litres to a cubic metre so a 30 litre rucksack is 1 cubic foot. Looking at retail websites and the range in capacity from 25-85 litres, you could say rucksacks come in 3 sizes (for medium-sized creatures) - small (1 cubic foot), medium (2 cubic feet) and large (3 cubic feet).
I think that works pretty neatly. Sacks would work the same way but Belt Pouches are probably too trivial to worry about. Don't have any easy answers as to the weight they could handle before breaking.