My pack is an older model Six Moon Designs Comet, unfortunately discontinued, a "little brother" of their Starlite. It's about 2700 cubic inches (~45 L) in the main bag and weighs 27 oz. with the "optional" (mandatory, IMHO) aluminum stays. I use the outside pockets a lot (tent in one side, water/snacks in the other, rain gear and a few other odds and ends in the front) but so far have never had to use the 400 cubic inch extension collar. I do have that space available should I have to carry bulkier gear. It has held up just fine to considerable use over the past 5 years. I did have to have some stitching redone after its first summer, but SMD did that for free! It has a Dyneema main bag and silnylon extension collar.

The ideal pack size, though, is the one that fits your gear plus the equivalent in weight/volume of a week's food for you and that feels good to you when you're carrying it for long periods. That's why it's important to get the rest of your gear first! If the pack is only part-way full, it usually won't balance well. The same happens if you have to tie stuff on the outside.