The original BYO article reported that they added a 10oz can of oysters to their oyster stout.
If you leave the shells on, you beer will become high in calcium from the shells.
I would suggest using only the meat of the oysters. Probably 16 - 20 oz, as you will not be adding the briney liquid from the canned oysters.
Primary: Dry Dock Breakwater Pale Ale, Raspberry Wheat, Cream Ale, Rye IPA,
Secondary: Brandy Oak Irish Red
Keg: Belgian Black & Mild, Golden Dragon 10.9%, American Brown Ale, Devil's Sin Amber IPA, Boundry Waters Pale Ale