Measure the volume of your gear.
Take all of your gear that you plan to carry in your pack and put it into a cardboard box large enough to hold it all. Remember to include your "loaded" food bag.
Put it in the box neatly leaving as few "voids" as possible.
Measure the length and width of the box and the depth of the gear in the box. Multiply those three measurements, L x W x D.
Determine what sleeping pad you will use and how you'd like to carry it. Do the same measurements and math on your pad without the box. L x W x Thickness of the pad. Add this result to the one you got from the "box" and you'll have your approximate pack volume needed.
If you use a hydration reservoir fill it and measure it in the same manner since it will be in your pack somewhere. Add it to the sum of the totals from above.
Add about 10 to 20 percent more volume, your call, for the extension collar to hold any extra gear. Now you have an approximate size of the pack that you need.
Pockets on the outside of the pack are "bonus" storage space for water bottles, shelters, rain gear and any other gear you want readily accessible. Don't include the pockets in the sizing of the main bag volume of the pack or you may wind up with a pack that is too small for your needs.
I hope this helps. Happy pack hunting!