Pack size can be a bit misleading.
A week long trip in summer is different to a week long trip in winter.
With that said, regardless of the length of the trip some things will not change.
This can be regarded as the "core" kit.
Shelter (tent/tarp/whatever)
Sleeping gear (Sleeping bag/kip mat/hammock quilt & underquilt)
Cooking kit (stove/pots/plates/fuel)
Personal admin (wash kit/FAK)
this kit is the same whether you're going away for the weekend or a month. only the amount of fuel in the cooking kit is variable.
Then we have the variable kit, which is mainly food and clothing.
Clothing can be washed on an extended trip, so that fixes that, as long as you have a bottle of travel wash and something to wash it in (a dry bag makes a great field washing machine) you can go on fairly indefinetly.
so the only real item that takes up extra space is food, and that comes down to resupply.
I can manage two weeks comfortably in 3 seasons with a 60L pack as long as I resupply every few days.
So, the real deciding factor on the rucksack size is your core kit.
If you have an expensive sleeping bag that can compress to the size of a grapefruit, and are happy to cook on a photon stove in a Ti mug, while sleeping under a tarp supported by walking poles, you could manage with a 45L pack easily.
If, however, you're using an army surplus green maggot and a tent from argos and cooking on a Trangia 25 borrowed from a scout group, you're going to need something bigger.
Don't get fooled into thinking you need a rucksack of X size. you need the pack your stuff fits into, and that depends on the size of your stuff.