I agree with Codger_64- I love the Baja Seal Lines. They're tough, which is absolutely essential. I've never understood people using ultralight materials for their dry bags. They don't last nearly as long, and you're saving a few ounces at the possible expense of waterproofing, which is the whole point of the bag. You can also extend the Baja bags' lifespan- you can decently repair any small holes that might develop over the years if you have duct tape and a lighter to really get that tape to bind long-term. I wouldn't try that with silnylon or some other super-thin material.
I wouldn't ever use anything above 25 litres, and my normal trip length is somewhere between one and two weeks. That'll hold a sleeping bag and as much clothing as you should ever need for a trip. It wouldn't also fit a tent, true, but I wrap my tent in the fly. While it doesn't make for perfect waterproofing, it'll keep it decently dry if my pack doesn't get immersed for longer than ten seconds. A wet tent isn't the end of the world anyway. It's definitely uncomfortable, so I see why some people might put the tent in a dry bag, but for me it's not worth the extra expense of a bigger dry bag. If your clothes and sleeping bag are dry, you'll have happy days and happy nights.
I also like to use a 10-litre dry bag for my first aid kit. I recommend a red bag for that particular purpose- just to make it a bit more intuitive- and I duct tape a cross onto it, just to make it more obvious.