First of all, go the Lite Gear Talk Forum and read the very first post, which I wrote, then follow the directions and ask your question again.

Second, since you didn't tell us where you are going or when, or what gear and clothes you plan on taking, I have no idea what size pack you need. That is why you need to read my post and follow the directions. I hiked all over New Zealand with a medium sized pack full of gear and clothes for that particular trip, but that pack would be too small for a winter trip, so the only answer to your question is "it depends."