The standard answer to what size would be: assemble the gear you will be taking before you buy a pack. The size pack you need will depend somewhat on the space taken up by your gear.

The standard second part of the answer is to take all your gear with you to the store and put it into the pack you are trying on. Because you may not be able to visit a store, the anaolg to this is to order a pack and do this when it arrives. You could get a reasonbable guesstimate ahead of time by putting your gear into a box and measuring the volume of the box.

Getting the right size pack is only half the problem. Getting as good fit is the other half and the more difficult half for some people.

Good luck!