I think you need to lay out all of your gear, tent, bag, clothes, cooking etc and see how much space you need. If you are an ultra-lite backpacker you can get away with minimal sized panniers but if you are not then you'll have to get larger panniers. I go relatively lightweight but I use a tarp not a tent, bag is down not poly, cook gear is small (boil water only) etc so I can get away with less space. Plus I dont mind wearing dirty clothes. Also what foods do you carry. Figure this out then buy some panniers