Take a look at the calculators on this page: http://www.v-cap.com/coupling-capacitor-calculator.php

If you take a couple of minutes to read the text and understand the 2 calculators you'll see why the values can vary depending for your situation. There is no "right" value as such, just a range as Martin said.

For me with 100k input impedance 0.8uF is recommended (I have 1uF in there). For a 10k input impedance you need more like 8uF.