If your planning to carry the essentials of day hiking for simple treks, then a pack that has 1,500 cubic inches or less capacity would be ideal. You might start running into problems if you get less than 750 cubic inches of capacity if part of your day hiking gear is warmer winter clothes or a rain suit. If you're planning to do weekend trips or lightweight backpacking then you should look for a pack with 1,750 to 3,000 cubic inches of capacity. This will offer enough storage space to carry your gear for a couple of days, or if your gear consists of ice axes, ropes, and other mountaineering gear get you up the side of a mountain for an extended day. Another reason to consider a small assault pack is if you plan to do extended desert hiking.
Water is very heavy, about nine pounds per gallon.
If you're going on a long trek you may be carrying nine, fourteen, or even eighteen pounds of water. A frame is going to help you carry that weight. If you plan to go on long trips of a week or more, or will be carrying gear for a couple of people, like a child or an inexperienced friend, then you will probably want the capacity of a full pack. Full packs range from 3,000 up to 6,000 cubic inches, or even more. They will have lots of room, lots of outside pockets, and lots of places to hang your gear.