How large pack you should get really depends on your style. Some people do multi-day wild camps with something as small as a daypack.
Personally, I'm on the other end of the scale. I prefer relative long periods without need for resupply, because it gives me a better wilderness experience. Also, I don't want to go hungry and I often cross areas where water is scarce (I have often carried 6-8 litres - this is outside the UK obviously!). And, I like my comforts. My longest hike was 5 months and then it is nice to have your tent not too small.
So, my choice was an Osprey Aether 70. Even with massive weight it still carries comfortably. I'm not winning any hiking speed records, which is actually the whole point. Just enjoy my time in nature and no worries