Your rucksack needs to be able to hold everything you need so that you’re prepared for the worst that any season can throw your way.
You’ll never regret packing something situational when that situation occurs. In the warmer months where you won’t need to be thinking about extra layers, we recommend a size between 20 and 40 litres depending on if you’re carrying for just yourself or some others as well. In colder months, you should consider something ranging from 30 to even 50 litres again depending on the activity.
For longer days in the hills at any time of the year you'll need your rucksack to fit more layers, food, etc than a short lower level trek perhaps, certainly for winter days then carrying extra insulation for a lunch stop or when weather turns nasty is essential.