While I was procrastinating one day awhile back (see Bishop Bob's Beads), I made a couple of miniature rosaries, one of which was this one:
It's made from ordinary #11 gold seed beads strung on sewing thread, a few synthetic pearls, and a tiny cross that I happened to have lying around. At two inches, it's approximately 1/10 scale (appropriate for a person six or seven inches tall ;-).
Periodically a spate of miniature rosaries appears on eBay as well, though exactly how miniature they are varies.
The most common version of "miniature" is actually about twelve inches long, made of somewhat larger seed beads than my mini-mini, and with a wire-chain construction. In fact, I own one of these, which I've had since I was about six.
(My very Protestant parents were not especially pleased with my interest in rosaries, but if I wanted to spend my allowance on them, they didn't prevent me. I remember my medium-sized clear crystal one cost me a whole dollar...)
These are commonly sold together with a case or locket, and can sometimes sell for as much as a full-sized rosary, even though they're usually of very cheap base-metal construction with thin cut-and-stamped medals that can seem scarcely thicker than tinfoil. (But of course you can't tell that from a picture.)
I've also seen tiny one-decade rosaries like this one -- admittedly, if there were a contest for "smallest rosary" these would have to be in a different category. The little purse on this one measures about 1+1/4 inches square.