The same goes for sculptors, of course. In addition, there are the limitations of the material being worked with, whether wood, ivory or stone. Some materials are hard to carve, some let the sculptor show more fine detail, some can better support being carved very thin without breaking. Weight, hardness, and durability are also factors, as is where the sculpture is going to be installed -- you don't have to be very fussy about details for something that will be mounted sixty feet up on the west face of a cathedral.
But from the examples I've seen, the rule of thumb seems to be that beads should be BIG.
My first example is a small ivory statue of Saint Rose of Lima. It's about 13 inches tall, originally created in about 1700 in the Philippines, and was sold by Sotheby's in 2002 for almost 18,000 Euros. Saint Rose was born in Peru in 1586 and was canonized or "sainted" in 1671. She is usually shown in Dominican monastic robes and with a crown of thorns on her head.
In this example she is also wearing a rosary around her neck. While everyone's Catholic grandmother has probably told them never, ever to do this, it was actually fairly common practice in the Middle Ages and Renaissance. (I'll talk more about this sometime.)
The string of beads is short, making a nice necklace length. The beads appear to be about as big around as her middle finger, which would make them about 14 to 16 millimeters in diameter. (Fingers are very useful for estimating the size of beads being shown.)
My second example is Saint Zita, or properly Saint Sitha in this case because that's what she was called in England and this statue is English. Sitha or Zita was born in 1218 near Lucca, Italy, and is the patron saint of housewives, servants, waiters, and people who have lost their keys :) She is often shown with a bag, keys, and sometimes a rosary.
This statue is carved in alabaster, a stone that is relatively soft and easy to carve. Alabaster was popular for statues because it is slightly transparent, so carved faces and hands can look remarkably realistic. Unfortunately it's also rather easy to break, so not many saints carved in alabaster survived the Reformation. This one is from ca. 1470-1500, and is now in the Burrell Collection in Glasgow.
Saint Sitha is holding a very simple rosary, just a loop of beads with a tassel. The carving of the beads is rather sketchy, but there seem to be about 40 of them and judging by her fingers, the beads are a little larger than Saint Rose's. If you look closely at the detail picture, you can see her keys (four of them) and a purse hanging from her belt, just to the right of the rosary.
(More about statues in the next post)