According to the 2000 edition BCA rule book: (covered rails) "Pocket openings are measured from tip to tip of the opposing cushon noses where direction changes into the pocket. This is called the mouth."
"Corner Pocket: Mouth........4 7/8" min to 5 1/8" max"
"Side Pocket: Mouth...........5 3/8" min to 5 5/8 max"