We call them coaming pads, never heard of carling. I would say 5" would be sufficient for most coaming pads. Have you looked into the Seadek pads? I would go that route if I were adding pads to my boat. I have been thinking about adding a pad between the leaning post and console.
http://www.gandermountain.com/modpe...oaming_Bolster_Pad_Set&aID=506E3&merchID=4006
http://www.gandermountain.com/modpe...oaming_Bolster_Pad_Set&aID=506E3&merchID=4006