I use a 65 litre pack, an ancient Karrimor Jaguar. More than enough for summer wild camps, and commodious enough to take a bulky synthetic 4 season sleeping bag in winter.
My eldest son is planning his first long distance walk, and has just ordered an Osprey Rook, 65 litres, I think, for just over £100. If I were in the market for a new bag, I would get one myself - they have a good reputation.
As long as the rucksack will compress down nicely, there is no harm in getting a large bag, even if it might be slightly under filled in summer on one night trips, when you will be taking less gear.