I have seen mantas at Midway, Yap and most recently just this past October at Sangalaki Island (I know I'm remiss in posting a trip report but hopefully I'll find some time now that the holidays is over).
Sangalaki was the best of the 3 for #s of mantas and also for size with the largest probably approaching 5-6 meters wing tip to wing tip. Actually, they have 2 varieties there - the giant Pacific manta (most comman) and also the pygmy manta (which I didn't get to see or at least didn't recognize). I'm estimating the size based on surface sightings and not the "enlarged" underwater view
We were able to snorkel with multiple mantas (20+ per snorkel) on every day while we were there and frequently saw them while diving also although usually not in those #s. On our 2 best days we did get to have "mantas on parade" while we were diving and had processions of between 20-30 mantas pass above, below and to both sides of us while we were diving. Its times like this that I wish my head would swivel 360 degrees.
I also have to mention the coral. I have never seen such an abundance and variation of pristine hardcorals anywhere (Midway, Hawaii, Guam, Yap, Truk, Palau, Sipadan, Okinawa, or Christmas Island).
The downside to the trip was the visibility which was 8-10m horizontal on the worst days and maybe 20m horizantal on our best days. There is a great abundance of plankton in the waters around Sangalaki which I'm sure has something to do with why the mantas are attracted to there but it does cut down on the visibility but a return trip to Sangalaki is something we plan on making.
Rickg