You might want to read the equipment specifications on pocket sizes, angles, etc. The full set is at
http://www.wpa-pool.com/index.asp?content=rules_spec
Below, I've quoted the relevant section on pocket sizes. These specifications are now for both WPA (i.e., international) and BCA play. The old BCA rules were a little looser; I believe up to 5 inch corner pockets were allowed (and, thus, up to 5 1/2 inch side pockets) and the maximum shelf depth was a little shallower. Note in particular, in contrast to what some people here have suggested about using parallel pocket cuts on the rails, the pocket cut angles are absolutely fixed at 142 degrees for corner pockets and 104 degrees for side pockets.
Shelf depth makes a big difference in the way pockets play, but there's not much you can do about it (sanely, anyway).
-- jwp
Excerpted from WPA table specifications (rule 9):
Only rubber facings of minimum 1/16 [1.5875 mm] to maximum ¼ inch [6.35 mm] thick may be used at pocket jaws. The WPA-preferred maximum thickness for facings is 1/8 inch [3.175 mm]. The facings on both sides of the pockets must be of the same thickness. Facings must be of hard re-enforced rubber glued with strong bond to the cushion and the rail, and adequately fastened to the wood rail liner to prevent shifting. The rubber of the facings should be somewhat harder than that of the cushions.
The pocket openings for pool tables are measured between opposing cushion noses where the direction changes into the pocket (from pointed lip to pointed lip). This is called mouth.
Corner Pocket Mouth: between 4.5 [11.43 cm] and 4.625 inches [11.75 cm]
Side Pocket Mouth: between 5 [12.7 cm] and 5.125 inches [13.0175 cm]
*The mouth of the side pocket is traditionally ½ inch [1.27 cm] wider than
the mouth of the corner pocket.
Vertical Pocket Angle (Back Draft): 12 degrees minimum to15 degrees maximum.
Horizontal Pocket Cut Angle: The angle must be the same on both sides of a pocket entrance. The cut angles of the rubber cushion and its wood backing (rail liner) for both sides of the corner pocket entrance must be 142 degrees (+1). The cut angles of the rubber cushion and its wood backing (rail liner) for both sides of the side pocket entrance must be 104 degrees (+1).
Shelf: The shelf is measured from the center of the imaginary line that goes from one side of the mouth to the other - where the nose of the cushion changes direction - to the vertical cut of the slate pocket cut. Shelf includes bevel.
Corner Pocket Shelf: between 1 [2.54 cm] and 2 ¼ inches [5.715 cm]
Side Pocket Shelf: between 0 and .375 inches [.9525 cm]