In the books, the Palantiri were a lot larger than in the movies. Here's a quote from a reliable source for all questions about Tolkien's Middle-earth:
Quote:
The palantiri were perfectly spherical with smooth, unmarked surfaces. They varied in size: the smallest were about a foot in diameter while the largest were too big to be picked up by one person. The palantiri were deep black in color when not in use and were made of an unbreakable glass or crystal substance.
[Source:
http://thainsbook.net/palantir.html ]
Furthermore, we have:
Quote:
The palantiri were kept in guarded rooms in high towers. They were originally placed on round, black marble tables with depressions in the surfaces to fit the stones.
[Source:
http://thainsbook.net/palantir.html ]
So the plinth from the movies is not the same as the original ones described in the book (Unfinished Tales, in this case), but it could be thought of as Saruman's change, if you like.
In Peter Jacksons Middle-earth, the Palantiri were indeed much smaller, and the size of the bead you picked looks good!