A recipe will often call for a "thumb" of fresh ginger, because the root grows in thumb-like chunks that can be snapped off from the root.
So how much is a thumb of ginger? While it obviously depends on the size of your thumb, a typical piece of ginger -- one that measures 1 inch long, 1/4-inch wide and 1/4-inch deep -- will give you 2 1/2 to 3 teaspoons of minced ginger.
Reference: How to peel ginger (Cookthink)
Reference: How to mince ginger (Cookthink)