The current default is 1 centimeter. This will be converted to whatever value that is in the current model unit setting is. I guess that is going to be a problem if you need sub-centimeter displacement in a meter model.
I believe the setting is limited to to decimals after the decimal separator. Is that correct @andy ?
Anyway, logged as RH-64214 Need more precision in displacement amount control (PBR)
I just yesterday posted a video showing how to do that with materials panel set to tree control:
I had no problem doing so in the video linked to above. Maybe repeatable steps with accompanying data would be good to have.