It really depends on the rest of the gear in your pack, as well as your level of experience backpacking. Do you have a tiny down sleeping bag or will you be using a bulky synthetic? Will you be sleeping in hostels, using a tarp, or sharing a 3 person tent between two people? Are you wearing most of what you'll be bringing, or do you like a bunch of extra clothes? Will you be carrying all of your food or will you be getting resupplies? Is this your first time backpacking?

I'm not sure if you mean wilderness backpacking, or traveling with a backpack backpacking, but for wilderness, 75L should generally be tight for up to 10 days of compact food in moderate weather with a lighter three season tent for an intermediate backpacker. If you are just using this for travel, and won't be carrying much food or shelter, 75L will be fine.