Well.. everyone is different is the thing.
You can look at the volume of the gear and supplies you're going to bring, and use that as a starting point. Some people eat more, or they love certain luxuries. Others need a bulkier sleeping bag, or they have a fancy ultralight tent. Bringing your compressed gear and inflated ziploc-style bags in to the store in a duffle to test out pack volumes and shapes can be a great tactic. (Maybe not the best plan when it's super busy, though. 😉 )
I'm personally a very ultralight type. I just have the lightest suspended-mesh back frame I could fall in love with, as the suspension was the most important factor for me. I strap things down on the outside as needed, and it handles any weekend I can throw at it.