I wish I still had my own old Triolet, but it wore out and I sold it in a garage sale.
To answer the original query
20 litres is a good around town and good weather day pack size.
30 to 35 for cold weather day trips
Mountain walking or skiing I always recommend something around 35 to 45 litres for the white season
Overnite/weekends that 35 to 45 litres is doable; especially if you have add-on pockets/pouches as I do.
Long good weather touring with room for all the needed stuff 60 to 75
Winter I use a 110 litre pack [ WE Alpine Expedition Large] and sometimes add all the external pockets [1* 8litres + 3 * 5litres] and the throat extension bumping it right up to about 155 litres at which weight and volume it is very hard for me to carry let alone ski with, but doable for short distances to set up a base camp
Ultralighters can knock 20% off those volumes for short trips; especially if the weather is kind and stable but at a higher cost in new UL gear
EDIT
Just remember you can carry a small amount of gear in a big pack [ with discipline of course] but it is difficult to carry a large amount of gear in a small pack
