lord of the rings - How big were the Silmarils?

I was reading through the Silmarillion and came up with this question:

Exactly how big were the Silmarils?

I don't see even a single reference to their exact size. The references that I do see say an elf could carry and throw one easily, which is pretty vague.

Another reference says that Morgoth carried all three in his hand and put all three in his Iron Crown. However, as a Valar he may have been much larger than human when incorporated on Arda.

For some reason, I'd always thought each of them was fist sized, but I can't say why.