You can change them in the drafting preferences (this so every time in that part you will have that font size)
Otherwise in the settings of the note/dimension.
Preferences Drafting->dimesions->text->here you have 3 types of texts.
Most of the NX drafting objects are kind of in a Static State. They won’t response immediately as you change drafting preferences. You have to select and change them or change the default settings and then apply them to selected objects.
Or else change the default settings and create new.