The smallest I'm comfortable making is a pound to a pound and a half.
I just got a dozen of the one-ounce sampler-sized FOs from Nature's Garden. I checked their info, and for most of them, their recommendation for usage rates is about 5%. And for a pound batch, that would be .8 of an ounce.
As I am really not interested in having a dozen teeny bottles with teeny amounts of FO left over, I'm going to resize my recipe so I can use the entire one ounce amount, which will mean a 1.25 pound batch of soap.
Anita