All typography settings you can find in Theme Settings -> Typography. So if you want to adjust for example Navigation, you would go to Theme Settings -> Typography and change default settings.
Font size is missing? Well it is not missing, Piwo used adaptive typography, it tries to be bigger where is can, and it scales down where it find it appropriate.
If there are some typography you cannot change it probably means we do not allow, why (?), usually what may seem like a limitation is for the comfort of the user and most likely results from a reason ie. responsiveness.
If you really need to make some custom changes, I would suggest using Child Theme. Make sure you back up before you modify.