This small and simple stitch is a very versatile one which is worth mastering. It can be used as an isolated stitch, in chains, as a filling stitch when bunched en masse or to finish a chain. It is frequently found adding texture to designs, or adding emphasis as an adjunct to other stitches.
A good knot depends on keeping a good tension on the thread, allowing enough space between entry and exit stitches so that the knot sits neatly on the fabric, and mastering the art of how tight to pull it. Size can be varied by the thickness or number of strands of thread, or the number of wraps. 2-3 is usual.
For good instructions on working this stitch go to https://sublimestitching.com/pages/how-to-french-knot
For more information, also check out https://pintangle.com/2012/04/17/french-knots-emboidery-stitch/
https://www.needlenthread.com/2006/11/french-knot-video-tutorial.html A very clear video
https://www.needlenthread.com/2012/06/isolated-french-knots-part-1.html Isolated knots
https://www.needlenthread.com/2012/06/isolated-french-knots-part-2.html Knot techniques