100lt is way too big.
If you need that much capacity, i suggest you take another look at what you're packing as you seem to have it badly wrong somewhere along the line.
Light packers are in the 40ish litre range as Sander said, heavier packers are maybe in the 60ish litre range. Going much above that can only mean you're taking enough haircare electrical appliances to run a salon and enough clothes to never do laundry - which is your choice but seems crazy to me. A good range of clothes and personal gear should fit in 60litres no trouble.
I am planning on a year or longer so you think 60ltr my mate said more like 80ltr ?
You should pack similarly regardless of how long you'll be gone. There will be plenty of places along the way to wash your clothes and carrying a bag that's above 60 liters is going to be absolutely killer on your back or a pain to roll around all the time. I even think sixty liters is a bit big, although it depends on what kind of traveling you're doing. If you're camping and you need to bring a tent and a heavy duty sleeping bag then sixty would be fine but if it's just clothes and things to get you by then I'd almost say stick closer to 45/55, just for how easy they are to carry around. It really will depend on what you need to take but I'd say 80 would be way too big. If you have the space then you will fill it up, which means it's going to me a monster, your back will appreciate you downsizing!