Manta Bommie is a good site to see the mantas up close, as they come in from the deeper water to this well known cleaning station, and the little cleaner wrasse get to work removing parasites and dead skin from the mantas. You can see a couple of the cleaners going into the mouth of the larger manta from about 1:00 into the video.
This was a very surreal experience, the mantas are a bit spooky and it took some time before they would approach us, while we waited on the bottom. After a few minutes it seemed like their curiosity took over, and they came much closer to check us out. At one point, the larger manta got so close that I had to pull the camera out of its way.
The larger one was about 3.5 metres wide, the smaller one about 2 metres.
This species (manta alfredi, or reef manta) can grow as large as 5.5 metres wide!