1.08 by 1.08 inches square as per instructions, with baille;
Sanskrit numerals form the magic square associated with the holy planet Jupiter and the holy number three.
There are yantras for the Sun, Moon, and Venus as well. All other grahas ("planets" in Jyotish or Vedic astrology) are also available by request.
These are available in many varieties of silver and gold, including very inexpensive plated versions.
For more information, see the book Numerology by Harish Johari. Here, here, and here are some more sites worth reading.