Determine how long you will be out on the trail. A trip that only lasts a few hours should only require a fanny pack (500 to 1,200 cubic inches), whereas a day trip will require a medium sized backpack (1,200 to 3,000 cubic inches). A multi-day trip will require a large backpack (3,000 to 7,000 cubic inches).
Consider the purpose of your trip and your needs. Photography supplies, multiple cooking utensils and tents will require a much larger pack -- 4,000 cubic inches or larger -- than a hiker with a sleeping bag and one cooking utensil who should be able to survive with a smaller pack of less than 3,000 cubic inches.
Determine the weather for the area that you will be hiking. Cold weather requires warmer and larger gear that necessitates a larger bag of 4,000 cubic inches or more, while warmer weather may not require you to bring as many supplies, allowing you to carry a bag that is 3,000 cubic inches or smaller.
