In reply to BURTON83:
It depends on many things including the nature of the route, the conditions, forecast, how well I'm climbing, my familiarity with the route / descent, how I chose to carry gear and even the risks I'm willing to take. Eg; If I'm climbing quickly and confidently at the grade, conditions and forecast are good I may well carry a lot less gear. So I may vary between a 25 litre to a 50 litre sack or even one pack between two.
Another factor is what gear you already own. Over the years I have learned ( still learning ) how to make more considered purchases and where I can save weight / bulk. For example, as a novice I usually carried a heavier sleeping bag if bivvying ( perhaps -5 rated ). Now I carry / can afford a much lighter bag as I have many more tricks for staying warm on a bivvy. So one way to find what size bag you need at the moment is to gather all the equipment you currently own and think you would take on a route and see what size bag that fits into.
The chances are, you will reduce pack size as you gain experience. I don't think there is one definitive answer.