The smallest one you can get by on! Or use a rolling suitcase unless you're going into the mountains and have to hike your gear. It's geeky but much easier to travel with and you won't hurt your back or keep whapping people with your pack on the train.
Bring as little as possible and do laundry on the way, or you will wish someone stole your bag so you don't have to carry it. Bring 2-3 pairs of jeans/pants, a few tops, 7 days worth of underwear/undershirts/tank tops and socks, one pair of sneakers or hiking boots/shoes, a pair of shoes in case you want to go out, a fleece and a waterproof or water-resistant jacket to layer over it, then do laundry along the way. You'll shed, trade, and buy along the way so keep it light and you won't torture yourself. Have a great time!!
