Tang, silk and cotton works well. Silk doesn't need washing, and I had good experience with double boiled / rinsed / dried cotton. There isn't an absolute amnt. for fabric; it depends much on the composition / weight and weaving. Do a couple of experiments; cyanotype chemistry is cheap to buy and/or prepare, don't worry much about the amount. You'll get the feeling very quickly...

Regards,
Loris.