Most of the ones I see are pretty large but I have some earrings that I guess are mabe pearls and they are only 8mm.

[jshepherd]

I do not actually deal in Mabe pearls, but I do know that the size is completely determined by the size of the nucleus attached to the inner shell of the mussel. This means that nearly any size should be possible. It is very possible that the 8mm pearls you have are actual mabe. They would not be extremely valuable, however.
Check out islandpearls.net. They have been producing some very interesting mabe pearls! The Chinese first used small jade Buddha nearly 4000 years ago to create intentionally shaped Mabe, and now we are doing it here for Disneyland!

We produce mabé, and the smallest size we have cared producing are of 8mm in diameter, but the largest size we've produced is 16mm... for a producer the target size depends on 3 factors: market demand, amount of mabé to produce per host-oyster and size of host oyster.

I've seen some very small (5-6 mm) mabé produced in small sized oysters such as the Caribbean Pearl Oyster species (Pinctada imbricata Pteria colymbus) and the Australian Arafura Oyster (Pinctada albina), but their quality is basically "experimental".

On the other hand, you have the larger White/Gold lipped Oyster Mabé (1.5 inch blister, 20 mm dome), but you can produce up to 16 (the most I've seen) per oyster if you seed them with the smaller 10-12 mm implants.

Regarding shape, you can use almost any shape, but the shorter the growth period the better the shape will come out...longer time (=higher quality) but the shape will soften enough to become unrecognizable.

One of the things we consider important for a Mabé is their HEIGHT. If the pearl's overall look is "flat" then it is mostly a "piece of nacre shell". Look at these examples of a Japanese Penguin, an Aussie Margaritifera and Sea of Cortez Mabé Pearls: