I'd start by listing what you are taking with you and seeing how big a bag it fits in.

Since you've never done this before, you will likely require less than you think in terms of clothes, both on and off the bike. When I tour, I take 1 pair of lightweight pants (trousers), 1 wool t-shirt, 2 biking wool biking jerseys, 2 pair of biking shorts, 2 pairs of socks, biking shoes and walking shoes. Oh, and 2 pair of quick drying underwear (pants). Are you taking more than that?

Remember that racks made of metal and panniers are heavy. The larger the bag, the heavier the bag.