Considering we're located in too-sunny-sometimes Phoenix, we sure sell a lot of dry bags in all kinds of shapes and sizes. Any customer kayaking needs to protect their phone, keys and what-not, and our backpacking customer need dry bags inside their pack to protect clothes and sleeping bags in particular.
For example, starting on the small side, we carry phone pouches with clear front panels that allow you to use the touch screen while still protected from water. Some have outlets for wiring as well. Slightly larger clear pouches can be used for print maps and tablet computers.
The 5- to 10-liter dry bags are sized well for a few clothing pieces or your down sleeping bags. 15-20 liter bags will cover about the same volume as you could get in a small day pack. Larger still - 35 liter and up - might hold nearly all your camping gear in a kayak or can line your overnight backpack if you are canyoneering or in a heavy rain zone.
Brands of dry bags we carry include Gecko, Sealline, Granite Gear and others - come check them out at our store or just give us a call for more details and availability!