I think you need to try to see a few systems 'in the flesh' to give you an idea of what works for you and what doesn't.
Don't be tempted to buy 'bigger panniers' as you will end up filling the space with things you don't need.
For what it's worth - my experience of a five week camping trip on my own - carrying cooker, tent, sleeping bag, food and water, tools, spares (air filter, oil filter etc), medical kit, clothes etc (i.e. could have carried on travelling pretty much as long as I wanted) I found two 38lit panniers plenty big enough and had a metal top box just for locking my helmet and gloves in (also had some paper maps and 'easy access' snacks)
For a similar trip two up I was grateful of one 45 lit pannier for the extra clothing (the other pannier was 38lit).
If you end up with one of the cheaper end of the metal box market, you might want to consider a waterproof liner bag (such as the Exped ones). Even if the pannier is waterproof, some of the designs are such that if you do have and off and the pannier distorts (or needs to be knocked back into shape) you've lost the waterproofing.
This also helps if you have one of the 'Touratech' style mounting systems which have two flywheels inside the pannier for removal. Having everything in a liner bag means you can just lift the lot out in one go rather than needing to unpack the whole pannier just to remove a it.
Don't know if that helps answer any questions, or just raise some more
Hope it helps
