(image taken from http://www.pololu.com/catalog/product/958)
The OUT line would be connected to a digital pin. Assuming you arranged a space between your phototransistor/diode and IR led for the coin to fall through you can get an estimate of the coin's size by recording how long the OUT line goes high when the coin is passing through (see the reference for the pulseIn() function). This assumes that all of your coins are falling through at the same speed.
Discrete photodiodes/transistors are cheap; I would suggest a plurality of detectors arranged in such a way that a coin rolling past them on a slope could be read for its height.