Good afternoon,
Welcome to the Apple Support Communities!
I understand you have a question about the font size changing when you are using the Facebook app.
In general terms, if an application supports it, enabling Dynamic Type can make it easier to read text on your iOS device. Here's how to check that setting:
Change the font size on your iPhone, iPad, and iPod touch
In some apps, the text size will get larger as you delete text, and smaller as you add text to a field. To see how the Facebook app works with text sizes, you can reach out to the developer directly:
How to contact an app developer
Enjoy your day!