About the sizes of the palantiri:
Unfinished Tales tells us that the minor palantiri (such as the ones of Orthanc, Minas Anor, Minas Ithil and probably Annuminas) were about one foot in diameter. The largest ones (those in Amon Sul and Osgiliath) were too large for one man to lift - quite big, I'd say. I'd assume that the one in the towers of Emyn Beraid was somewhere in the between.
About the history of all the palantiri:
I do not think that the palantiri were counted as "jewels". They were more like "crystal balls" and I see no reason to assume that they weren't used by the Noldor when the stones were first made. They were tools of communication after all, and the Elves prabably kept one in Tuna for the Noldor, one in Formenos for Finwe and others of his house, one with the Teleri in Alqualonde, one with the Vanyar in Valmar and so on...
Now why were they given to the Faithful (Elendil, to be exact) and not the Kings of Numenor in earlier times?
While I believe that originally there were more palantiri than just seven in Valinor, I don't think that they were a very common commodity even there. If the palantiri really were made by Feanor, then they were irreplaceable (is that a word?) too, and ones which were given away from the Undying Lands were permanently lost from the Elves. So, they can't have been given just out of pure generosity (it would've been far too great a loss for that), but the Elves must have known that Elendil and his followers were in deep trouble and that they needed secret means of communication in order to avoid the inquisition - and the palantiri were just right for that.
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