I am planning to go on a 2 week touring trip in a few months and I'm looking for some bags/panniers to buy.
What are your recommendations regarding the capacity of the bags (considering the length of the trip)?
Also, do you have recommendations for any specific bag?
EDIT: I will pass through towns or villages each day, so I don't need to carry a lot of food/water. I will be carrying a tent and a sleeping bag, some tools and clothes.
Short answer based on the little information given I'd guess two larger panniers would be enough, especially if the tent and sleeping mat is strapped on top. Watch out for the heel clearance, especially with the largest ones.
Long answer and rambling: The length of the trip doesn't matter as much as you'd might think. It really depends on how much you'll going to pack. The truth is that probably a beginner will back more than an experienced bicycler tourer because you have to figure out for yourself what items and luxuries you'd like and which ones aren't of value to you.
Make a list of the things you would like to take along. Maybe get some feedback from experienced tourers and have them review and comment on your list. Take their suggestion with a grain of salt, since it's really personal what one takes along. Then judge from that how large of a panniers you need. Don't over stuff them if possible. Not only will it be harder to get your things out of the bags but you also need some room for food purchases during your trip. Often people stop at a grocery just before the campsite to stock up on food for the night.
Don't carry water in you panniers. Two or three water bottles in cages should be plenty. You wan want to take a small water bladder along if you go through the occasional area where extra water is needed (i.e. campground where drinking water isn't available).
Ortlib and Arkel are some popular pannier bags but really any sturdy pannier bags will work. Some people even mount kitty litter tubs as pannniers to their rack. Ortlib bags are by default rain proved but they also don't have external pockets and aren't breathable. Most other bags including the Arkle need a rain cover for rain. BTW, try choosing a color that improves your visibility (yellow or red instead of black).
A main thing to keep in mind is to balance the weight evenly between the sides. If carrying much you can also mount a front rack. This will also distribute the weight off your rear wheel. The goal with front racks is to keep the center of gravity low. So lowrider front racks are often preferred.
Some alternatives to pannier bags are trailers (single wheeled like the bob trailer or two wheeled child trailer), frame bag (especially popular with mountain bikers in conjunction with handle bar rolls when riding narrow trails), or just tying everything onto your rack.
[links are just for clarifications, not for endorsement]
It can be all over the map, from a backpack containing a sleep sack and a credit card to a trailer hauling everything including the kitchen sink (a fold-up one). I recall one guy who did a week-long trip with only a sleeping bag, a "bivy sack", and a pair of tennis shoes bungied to his rear rack.
I'd suggest you first study what racks can be attached to your bike. If you can attach front racks, that's goodness. There are special racks with latches for matching bags, but I'm not sure that's necessarily a good idea -- nice to have choices.
Then two large standard (but good quality) rear panniers and a couple of medium-sized front panniers should be fine for most people. (Even if you don't need the front panniers for space, placing about 25-35% of the weight in the front makes the bike handle better, in the opinion of most.) The bags should have at least a few external pockets, and sometimes bags with internal partitions work out. Note that tent, sleeping bag, and/or sleeping pad can often be strapped on the outside, but be sure they are securely strapped on, since a loose tent or whatever can start vibrating badly on a fast downhill.
Not sure who the best pannier makers are these days -- Cannondale used to be pretty much the best, but they've all but quit the market. I don't do any self-contained touring anymore so I replaced my Cannodales with a couple of small no-name units when my Cannondales finally wore out.
You will want two large-ish (30-40 litres each) panniers in the back, and two small-ish (20-25 litres each) panniers in the front. Having them on both ends of the bike improves handling a lot, and it lessens the pressure on your rear tire, which can save you quite a few flats.
I have two Ortlieb back roller classic panniers in the back, and two front roller classic panniers in the front. I carry my tent on top of the rack, and the rest in the bags. These offers more space than I need for a two week trip, but they squeeze smaller pretty easily, and there are always times when the extra capacity comes in handy.
The most important point of these bags is that (when properly closed) they are completely waterproof. If you only bike in sunny weather this may not be an issue, but if you expect rain then the certainty of a dry change of clothes and a dry sleeping bag to crawl into at the end of the day is a the best morale boost there is!
