From reading 'Regulations' on the relevant Wikipedia page, it appears that the smaller 'Royal & Ancient' balls ceased to be acceptable in 1990, when the larger 'USGA' balls became the only balls for use in competitions:
http://en.wikipedia.org/wiki/Golf_ball
I don't know about the official view but I haven't seen a 1.62 inch diameter ball for sale in more than 30 years. I think the 1.68 (American) ball was phased out in the 1970s. Certainly for tournament play.
Sorry Brionon. My post should have said the 1.68 American balled was phased IN not out in the 1970s. Apologies. The 1.62 diameter ball was easier to control in windy conditions and required less skill to shape shots.