I believe you should carry two fixed blade knives in you Bug Out Bag. I carry two in case one is lost or fails me. Another reason why I carry two is each one has their own job. I carry the Ontario Air Force Survival knife, its cheap and tough. It is made from a softer steel which will bend and give instead of chip and break. Using it for more rugged task. I also carry the Benchmade 158 CSK-II because its a good all around survival knife. It holds an edge and has a 6 inch blade. The Benchmade 158 CSK-II is also a good self-defense blade. Since these stay in my bag and are never used, I tend to keep less expensive knives in the BOB.
I would stay around 4-6 inches on your Survival blade. Larger knives do serve their purposes but they have limitations in survival craft. A 4-6 blade can accomplish most things that you will need to do.
Another item I always carry is a good quality multi-tool. They come with a variety of tools that keeps you from carrying multiple items. I also use the small knife blade on the multi-tool for cleaning fish or game and other small task. They can open cans, saw limbs, screw and unscrew things, and file to name a few.