Everyones BOB will be different, as it will depend on what your plan is. For me I have more of a get home bag, that I carry in my trunk. If/when the SHTF then I plan on holing up at my home, (out in the country). I just have an ALICE pack with just enough stuff to get me home (about a 50 mile trip). The military has been humping around with these for years (I know I did my share), and they do have different sizes available. And, unless you plan on backpacking all the time then dont worry too much about the high dollar fancy bags, because its just going to sit in a corner somewhere collecting dust.
You may also want to consider fatigue, it may be better to have a few small packs for your family to help carry than one big one that will wear you out faster, and put you in a position to not be able to function at 100% if you need to respond to a threat, or some other situation where your fam will need you.
Not advertising for another forum... but... check out zombiehunters.org they have lots of information and discussions on this topic, plus they will report back on the equipment that they have and have tested.