I've always just gone with 1/4 inch. Many people sending scion
wood will try to get "pencils" of wood, and pencils are a quarter of an inch in diameter. Additionally, 1/4" offers a bit of flexibility since if you get appropriate sized scions you can do a saddle graft or a whip&tongue graft, and if you can only get thin diameter twigs of a variety you are looking for you can use a cleft graft into the 1/4" rootstock.
In terms of how much to get, rootstock is one of those things where you can really see the economies of scale. For example, if you were to order 30 M111
apple rootstock from
Fedco it will cost you $87 plus shipping. At the same time, if you order 100 M111
apple rootstock from
Copenhaven Farms it will cost you $97 plus shipping. With that in mind, I would suggest ordering based on the area you wish to fill,* and there are worse things in life than having to give
trees away to your friends and neighbors if you order too many.
**One caveat to that would be if you are ordering pear rootstock and will be growing them in a nursery area for a year or two before planting them, order 50% more than what it will take to fill the area you want filled. I've had terrible luck with pear rootstock.