It's kind of hard to tell how much space you need without experience. And you need some sort of pannier to get that experience.

If you have experience with other kinds of self-sustained, space-constrained travel (eg, backpacking), you have a reasonable place to start from.

If you have really big panniers, you might be inclined to carry more stuff than you need.

Starting with the stuff you plan to carry is a good idea. It's possible that a outdoor store that has panniers might let you try-out packing what they have in stock. Whether or not you buy what they provide, it would give you an idea of the sizes of things.

Keep in mind that you might want some reserve space for carrying food, etc. That is, don't start out with the panniers stuffed.