There are plenty on this forum who eschew the need for any more than 2 panniers for just about anyone. So if your wife wants only rear panniers, it should be quite manageable, especially if you are willing to carry the bulkier shared items like the tent and cookware. I have found that my standard touring gear can fit into two, large panniers and a bag on top of the rear rack. I don't do that because:

• I prefer to carry more weight in the front than in the rear.
• It's easier to divide up my gear by function.
• It leaves the tops of my racks open for stowing temporary items.
• I can pack more crap (maybe more of a con than a pro).

But like I said, two bags and a rack pack are the preferred kit of many, so it shouldn't be a problem. A decently-sized handlebar bag, and there should be plenty of room. Heck, if someone else would carry my tent and my cooking gear, I expect two panniers, no rack bag, and no front bag could be easily managed.

As for dry bags/compression sacks, it largely depends on how your carrying your gear and whether or not your panniers are waterproof. You definitely want to keep the sleeping bag dry. You probably want to keep it compressed, especially if you're trying to keep your wife's load volume down. I am also a fan of the Sea-to-Summit, water-proof, compression sack. I have a fairly large one into which I put my hammock tent, sleeping bag, pillow, and sleeping bag liner. The whole thing gets squished down and squeezed into a fairly large pannier. Another dry bag holds my clothes. If my panniers were waterproof, I would probably worry more about compression (for my tent/sleeping gear, at least) than waterproof, but even then, I doubt a little redundancy would hurt, especially since I don't find my Sea-to-Summit bag to have much of a weight penalty compared to a non-waterproof compression sack.

The size of your dry bags/compression sacks depends on what they need to hold and what needs to hold them. My bag, uncompressed, has a larger volume than my pannier, and compressed, it can just squeeze in. But for my clothing, I have a smaller bag because the clothes take up less volume and share pannier space with other items. They also aren't really compressed. Their bag is waterproof and has the Sea-to-Summit, air-venting fabric, so I can get it pretty compact, but my clothes don't take so much room that I'm worried about compressing them. If I needed to worry about cold weather travel, that might change the equation. Still, I like not compressing the clothes simply because it makes getting into and out of the bag less hassle. The sleeping gear is only pulled out/packed, once a day, so I don't mind, but if I don't need to, I'd rather not deal with compression straps every time I need to shed or add another layer.

My suggestion: First get your packing list. Then figure out your packing strategy--what goes where. Then figure out your bag needs based on whether the gear needs to be compressed, kept dry, or both. Dry bags should be have just enough volume to cover the space where they'll be carried, but not bigger than they need to be. Compression sacks you can over shoot that mark with the idea that they will compress down to fill the space, but still don't go crazy. A large compression sack won't shrink down tiny just because you didn't put much in it.