What size camping pack do you need for a weekend camping trip?

That depends on a lot of factors. Presuming you are going to backpack your way to your camping site, you need to take into consideration how much clothes, gear, food, etc. you will need. Generally, you should be good with a 30-40 L bag, presuming you do not bring your whole house along with you. However, if you will use it for longer trips, you will need more room, say a 50-60 L bag.

The main thing you need to look at when you are buying a pack is the fitting, you will carry it or say 7 or more hours, so it must be comfortable. Packs generally come in different lengths. Make sure your pack is the corresponding size to your back. This limits the size of your bag.

Finally, walk around in the store, go up and down steps and ask the sales person for advice. In certain stores, they can put weights in your pack to test out the fit. Your pack will become your best friend, so chose wisely!