Biggest thing with a rucksack - it has to fit properly and be comfy!! Size, as people have said, you will fill the size you choose. So put out all the things you think you need, discard the things you put there unnecessarily, and look at the pile you have left. Figure out what else you can lose, make sure you have basic survival kit, and then buy a sack big enough to take it.
I carry a daysack normally, for the odd day out. I have a montane grand tour (65, I think), for longer walks, but nearly 50% is food for a week, and extra first aid etc as I am on my own.
But make sure it is comfy ... you will be up close and personal for an extended amount of time
And, no matter how well you think all this out, in 2 years time you will be looking for a different size .....