When I for example wash my face does every inch have to be washed or is it ok if you make a mistake and leave a few inches?
Performing some of the acts of wudu' three times actually is recommended in order to solve such cases. This is based on the practice of the prophet () see for example in al-Muwatta' and Sahih al-Bukhari.
Washing your face during wudu' is a fard (ordered) act, as Allah says:
O you who have believed, when you rise to [perform] prayer, wash your faces and your forearms to the elbows and wipe over your heads and wash your feet to the ankles. ... (5:6)
so you should take it as serious as possible and try to let water reach all of your face, your beard (if you have any) etc.: the whole face from ear to ear and from hairline to your neck should be covered with water after performing it.
This means: If you found out a place which water didn't reach while performing wudu' you should correct this else you may expect having performed your ablution well. Further "wash" means the water should "run" in opposition to "wipe".
Every inch between fore head to end chin. and between one ear to another must be washed and its better to wash three time so that not a single inch left un washed.