This article is from the Satellite TV FAQ, by Gary Bourgois email@example.com with numerous contributions by others.
For strong subcarrier signals, a smaller dish will work quite well. For
general reception of most wide band and some narrowband signals, a good 6
footer, such as the new Orbitron 6 foot dish with MICROMESH will provide
perfectly satisfactory reception. 4 or 5 foot dishes will give good audio
performance on wideband audio carriers and if you can accept some noise, a
cheap 3 foot dish would at least provide you a signal on BBC or other strong
signals, though there will be some hiss, it will still be better than
shortwave reception. Experimenters claim to have gotten decent audio with a
two foot dish. However if you live on the outer portions of the USA
(Florida, California, Washington State, Maine, etc) you will need a bigger
dish to get the same quality as someone in the central USA in most cases.