Bicycle touring with panniers can be different than recreational riding with a trunk or small rear pannier. Regarding size, you will need over 2,000 cubic inches if on a 2-3 week cycling journey. And if camping you will also need front panniers because of the extra gear such as a tent, sleeping bag and cooking equipment. Some cyclists like both front and rear panniers anyway just for the balance.
When packing you need to balance the weight in the various by first putting the heavy things in bottom. Items such as snacks, bike tools, rain jacket and camera need to be at the top or in a handlebar bag where you can grab them quickly.
I place the other items such as toiletries and clothing in a series of plastic bags (even if you have water proof panniers) so you can pack quickly.
Some of my friends even practice packing and riding with loaded panniers before a journey, although I never have. One thing I do is pack everything in the same place every day before setting out so that if something is missing you will notice it.
I find that anyone on their first cycle tour tends to overpack. On one European trip several people mailed some items to our final hotel. On my first bike tour I ended up throwing some things away after the first day on the road.
You may have pannier covers if your bags are only water resistant like mine. These are actually handy for tucking small items under that you may need to get fast like a jacket or vest.
I always ride with a small group of friends so starting out the first day is interesting as we are not used to the weight and wider loader and sometimes bump into each other. But after a few miles along the bike path you do get used to riding with the panniers and the others.
For a 2 to 3 week self-guided and planned bicycle tour I take two rear panniers and a handlebar bag only and travel light. We stay in small hotels or Bed & Breakfasts so there is no camping equipment.
When stopping for lunch we try to find a park (if picnicking) or a restaurant from where we can see our bicycles. Although the bikes are locked the bags are not. We have never had an issue in years of bicycle touring in Europe and North America.
However when we stop for the evening, using the quick release and the carrying handle at top we take everything into the B & B or hotel. In Europe many hotels have bicycle lockup rooms and generally someone will lead you there first before you head to your room.
Hopefully you have a excellent mounting system so your panniers won’t come off when you hit a bump (or cobblestones in Europe) along the way. On the plane heading to the destination I bungee them into one big bag (1 piece of luggage) with my bike being the second.
Click the link to check out various styles of panniers for cycle touring. It is amazing how quickly you get used to bicycle touring with panniers.