not having done much touring since the early 70's, I can only say things that are similar... If you are a backpacker, the larger the pack, the more you will carry, as you will tend to fill the pack. The same applies to panniers on a bike. Figure out what you need to carry, then re-analyze and THEN buy your bags. I am always amazed when I come back from a week long bike trip (where I don't carry y stuff on the bike), and realize how much I took with me that I never used. Obviously the weather is a big factor, as you need dry clothes to get into if you get wet, or ride wet; you need cold weather gear in some areas and so on.
train safe