The inside diameter of adhesive ring should be about the same size as, or larger than, the diameter of the electrode contact.  The inside diameter of the adhesive ring should not be larger than the outer diameter of the electrode housing.  The outside diameter minus the center hole determines how much surface area contacts the skin, thus determining how tightly the adhesive will adhere to the skin.  Also, the outside diameter will limit how close you can place electrodes to one another and to other features, such as the eyes.
Using ActiveTwo as an example, the electrode contact on a flat-type electrode has a diameter of about 4.5 mm.  An adhesive ring with 5 mm id (center hole) would be  idea, but the 5x13 adhesives are rather expensive because they are manufactured in Europe.  We recommend using a 4x19 or a 4x12 adhesive ring.  The 4x12 is a good choice when placing the electrodes close to one another or close to the eyes for startle measurements.  The 4x19 is a good choice when you have plenty of room and the primary concern is how well the electrodes stick to the skin.