An excellent resource regarding bug out bags is a new book by Max Cooper called, “Realistic Bug Out Bag, 2nd Edition: Prepared to Survive.” This is a monster book at over 600+ pages. It has scenarios, drills, and is full of useful and insightful information. I like that the author stresses planning and has a section devoted to bug out plans and how to practice & train your plan. He is also a huge advocate of designing a BOB that fits your needs based on factors that pertain to your situation. I highly recommend this book.
I’m using an Osprey Atmos 65 backpack in large, which gives me actually 68 Liters of space. I chose this particular bag because I already have an idea about how much space I need from using my Camelbak BFM for so many years. I originally got a 5.11 Rush 72 backpack to replace it, after MANY people suggested it and read some really great reviews on it. After thinking it all through, it wasn’t going to work. In order to carry the amount of items I wanted, I’d have to add cargo space to the bag and it already weighed quite a bit. Plus, it’s a very military-looking bag, which I’d rather avoid if I could. It also seals up at the top and has a built-in cover for the rain.