I climb the Lake District mountains twice a week,and find 20L is the ideal size.
With its Airflow back system,which is unusual for this size,the ability to separate,or not,the top and bottom sections of the pack,and the waterproof cover,these are very useful specifications.
The exterior bottle carriers, which can also be used to carry wet overtrousers,and outer pockets,to carry smaller things you can reach without rummaging in the main part of the pack make this an ideal piece of kit.
And cheap too !