When climbing Kilimanjaro, you will need to bring at least 2 bags. One should be a daypack which you will carry during your hike. The second bag can be a duffle bag or a backpack. This bag should be slightly larger, from 75L to 90L.
Think of your first bag, your daypack as your carry on luggage. You will have access to this bag during the entire day as you hike. This daypack will weigh about 15lbs. It will contain your water, your lunch, snacks, raingear, camera and maybe an extra layer of clothing.
Your second bag is like a check-in bag. You will give this bag to a porter and he will carry it from campsite to campsite. You will not have access to this bag during the day. It will be waiting for you inside your tent when you arrive at your campsite. This bag will contain all your other clothing and toiletries.
The second bag should not weigh more than 30lbs. If it is a duffle bag, it does not need to be waterproof as the porters will put it inside another larger waterproof bag. If it is a backpack, it should have a rain cover because the porter will most likely wear it as a backpack as he carries it.