There is no one height. The idea behind the air dam is to reduce turbulent airflow under the car and redirect it to the sides where it hopefully less turbulent. Many use a measurement that coincides with the low hanging parts. On my GP thats about 6".
In this picture you can see the subframe in the background my air dam is 1/4 inch below that. This configuration on this car vary rarely scrapes because it is further back. If it was forward of this location it would drag more often. Same with the side the wheel base will determine how low you want go.
Many here have done lawn edging air dams. Do a site search there is quite a bit of information on air dams and splitters maybe even some for a Fit. You might also want to look in to belly pans.