Minimum pack size is completely determined by the gear you carry. That's why we keep saying to buy it last. Assuming the manufacturer is accurate in their listed pack volumes you can calculate the volume of each piece of gear you have and add it all up. Then determine how much room you'll need for food and water and add the two together and you'll be somewhat close to the minimum pack size you can use. Go a bit over though so you don't have to over compress your insulated gear.

Or you could post up a comprehensive gear list and people here may be able to tell you the size range you need to be looking for in a pack. Remember to include how much food and water you need to carry per day you'll be out AND what kind of food it is.