I think dry bags are overkill if you will be putting them in panniers. A trash compactor bag or gallon ziplocs suffice just fine. I use dry bags when foregoing panniers though.

A tent in my opinion belongs on the top of the rack and requires no dry bag. I just roll it with the waterproof floor on the outside and it stays just as dry (or wet) as it was when I rolled it. Your sleeping bag or pillow must be huge to require 13 liter and 8 lither dry bags unless you aren't packing them tightly at all.

As far as sizes I really can't say because that is dependent on your gear. Personally I find that very surprisingly gear is required and it can be compressed to pack quite small. These days my gear all would probably fit in the two bags you use for just the sleeping bag and pillow. I'd advise getting your gear together and then buying whatever bags it requires. In the days when I packed more I found that panniers and ziplocs were the way to go and when I got below 20 pounds of gear I found that the dry bags without panniers worked very well and below that the lighter and more minimal the gear the more it made sense. I have found that it can be comfortable to camp and cook with 15, 10 or even less pounds of gear if you think out the gear very carefully. I now much prefer that approach.

If the notion of packing lighter appeals to you check out my article on the subject. Also if you follow the link in my sig line you can find a lot of info on my tours including the early ones where I packed heavier, I tried to include useful details in all of the,

Either way have a great time touring.