What are the size ratio's of this simple mechanism?

i am modeling a clock with a simple 3 gear system. the largest gear turns the hour hand, the one directly beneath it is the one that rotates the minute hand. the final gear is placed beside the other two and is meant to transfer the rotation of the bottom gear into a smaller scale rotation. this process takes the faster rotation of the bottom gear and makes the top gear rotate at approx. 1\60th of that speed. if the top gear is approx 20 in. then how large should the transfer gear be?