I usually use embroidery floss and French Knots can be made with anywhere from one to all six strands of floss. The number of strands used will help determine how large the knots are. Another factor is how many times the thread is wrapped around the needle. One of the most helpful things I have done is make a grid of French Knots using from 1 to 6 strands of floss and one to four wraps. This little grid is helpful when deciding what size knot I want to fill in an area or create a round counter point. You might consider making one for yourself to keep as a handy reference. I can't tell you how often I use it - but a lot! Here is what it looks like.
Here are some of my favorite ways to use French Knots.
To fill in areas like the sheep and treetops.
To fill in for texture in a background or border.
To enhance other stitches and create the center of flowers.
Another example of adding them to a set of stitches.
If you have not tried French Knots, they can be a little tricky but here is an excellent video as well as some stitching tips from the fabulous Mary Corbet of needlenthread.com.
I thought I'd end with bright pink French Knots exploding out the ends of this flower. Happy knotting!