Reddit mentions of Sea to Summit Ultra-SIL Stuff Sack, Royal Blue, 2.5 Liter

So what I'd say for 40 vs 50 is, first of all find out how much space you'll need, then find out the weight for each bag. Depending on how the bag is built, there's a possibility the 50L could still be lighter and it might also be possible (depending on the bag) to tighten the straps to make the 50L bag into a 40L size. Really, much of it depends on the bag and how it's built.

Just because you buy a 50L that doesn't necessarily mean you need to fill the whole bag either, it's always nicer to have a bag with lots of extra room inside as it makes removing stuff from the bag easier plus you may come across a souvenir you just must have. Personally I keep all my stuff in a bunch of stuff sacks (like this so it's easier to remove stuff from my bag. But that's because my Osprey has barely any pockets or sections to it - which is a pro [lighter, can squeeze down more] or a con [harder to access certain items] depending on how you travel. I also have small canvas bag (kind of like this one) which I keep in the top of my backpack. That way, I can drop my backpack off at a hostel or wherever, and still have a small bag to take out with me to hold a book or whatever. And it's lightweight and easy to push into another bag.

For me personally, my 38L is the perfect size, but as I said, it's really about how you travel. I don't know if I'd like to go any bigger as I like having a bag which is easy to throw into a taxi or take on day trips. My bag can be squeezed down with straps too so that it becomes much smaller. But I've seen a lot of large bags that stay large in size, even when they're empty.

I can't really point you in the way of a decent brand because as I said, I've been using my own Osprey bag for 8 years and have needed nothing else!