Unfortunately this first version of BottomNavigationView came with a lot of limitations. And for now you can’t change titles size just using the support design API. So to solve this limitation while google doesn’t implement it, you can do:
In your dimen.xml you can put:
<dimen name="design_bottom_navigation_text_size" tools:override="true">30sp</dimen>
<dimen name="design_bottom_navigation_active_text_size" tools:override="true">30sp</dimen>
Doing this you are overriding the default value of dimen that the internal classes of BottomNavigationView use. So be carreful.