Just to start things off:

Depends on how much "comfort" you require. You could manage fine with a fairly small pack if you are OK with being a bit hungry and cold and improvise along the way. Most people would prefer an "ordinary" sized pack (now enter the interal frame vs external frame debate) in the 3500 ci - 5000 ci range.

If it's an all-up BoB, you need to size it to include your winter clothing and gear. If you are OK with a seasonal supplemental bag for your seasonal clothing, a smaller bag will do fine.

7 days food adds up to a lot of bulk and weight, no matter what you use. Figure how you're going to handle water and assume surface water in the MidWest is full of runoff from livestock AND ag chemicals (because it is).