So far, we haven't discussed any of the features and benefits of daypack design.
Let's use this
one for an example.
First thing I'd look for is two outside pockets that will each hold a water bottle. Most of us just buy Arrowhead water or some other brand and use those bottles. They are lightweight and come free with water.
Then there is the compartment for lunch and snacks along the trail. This is normally towards the back of the pack. I'd also put my water filter in this compartment.
There is the small zippered pocket on the top which is a good place to put your wallet, car keys and minor first aid kit.
The outside pocket is a good place for a field guide, journal, or whatever else you might want quick access to. I'd suggest keeping these in Ziploc bags.
Finally, there is what I call the big bag section where I put my extra clothes, stove and fuel.
Saying what size will work is difficult as it may be dependent on how big you are and what specific clothes you bring.
My suggestion is to get all your gear together and go to REI if you have one in your area. I'd stay away from the box stores.
If you have to order by mail, REI has a very liberal lifetime return policy. If you become a lifetime member for $20, you will get emails on sales, a 15% dividend on many of your purchases and you don't have to keep receipts.