What is the average opening of the mouth and jaw? In an adult? A man? A woman? A child?

Most studies show that the maximum jaw range of motion (ROM) or maximum mouth opening (MMO) is related to body size and height. So men can usually open wider than women, taller people more than shorter people. In studies, MMO for adults has generally been around 50 mm, with a range from 32mm to 77mm. Men can open to about 50-60, and women to 45-55mm.

In a study done in 2008, linked below, a team tried to determine how to assess oral opening function of children. This is difficult because they have such varied body sizes and growth rates. In their study they suggest using the 3 finger test to simply and quickly determine the maximum opening. In this test, patients use their index, middle and ring finger of their non-dominant hand as a general gauge for a range of their opening, but they also note "it is not highly reliable"

Want more info? Click through to the study abstract:

https://www.ncbi.nlm.nih.gov/pubmed/18686495