I always pictured them as being about the size (and approximate shape) of half a golf ball. Maybe a tennis ball.
If they were intended to be jewels to be worn, they would need to be shaped in such a way that they could fit into a setting that could be comfortably worn. And the silmarilli
are frequently described as being worn -- in Morgoth's crown, in the Nauglamir, on Earendil's brow, etc. Given Feanor's ego, I always felt that he would have wanted to be able to wear his greatest creations, and would have made them in a shape (more or less hemispherical) easily placed in a setting. Tolkien's drawings of them in his heraldic devices shows them as looking, in shape, like six-pointed stars in circles, which could be his way of indicating that they were also faceted.