I read accounts of people doing it with both 1.5" - 2.5" holes... Unfortunately I didn't see any explanation of why a certain size should be used... the only thing I read about size of the holes was that the larger total "hole space" (combination of the size of each hole and number of holes) the more sound could pass through and the more cancellation would occur. This is definitely anecdotal, so if you find anything that conflicts with that advice, definitely go with that source over what I remember.
Check
this article on Audioholics for a good review.
