Hawk, start with it small, shoot the gun in various light conditions, and see how it works for you.
There's no set formula for this. You have your eyes and no one else's. The rest of us can give you general guidelines -- I personally like a bit more light to either side of the front blade, when peering over the sights, and I use a square notch rather than a V (and this is with what the last eye doctor I visited called "better than 20/20" vision -- whatever that means). How well you shoot, though, is the only real standard that ought to determine whether your sights are right for you, or not.