To answer your size question, 10". I'm not a fan of advising people to buy a 12" because most won't use them as often. If you cook at home in a big crock pot or a 12' oven dutch for a big family, then you might use one, but most won't'
We have a 2 8", a 10" and a deep 12'. The 12 doesn't see much use except when we have the kids and GK's around, but that's a crowd 8 people.
The 8's get most of the time. There are generally 3 adults and they are plenty big enough. They are also a perfect size for cobbler, 1 can of pie filling or a big can of peaches topped with a Jiffy cake mix is all you need. The same can be done in a 10".