Can font size be changed without leaving text message.

Perhaps you may try to see if that is an accessible feature with your iPhone model and its iOS version.

..Did you view and read the Related Article, above your post? ~ Those often have some clues..

"To learn more about the 'accessibility features' and how to use them,

refer to the accessibility section of the user guide for your iOS device."

Another Link to see the 'related article' page is within blue text above; or spelled out here:

Change the font size on your iPhone, iPad, and iPod touch - Apple Support

//support.apple.com/en-us/HT202828

Linked iPhone User Guide (table of contents) ~ shows much more than you ask.

iPhone User Guide (Table of Contents) - Apple Support

//support.apple.com/guide/iphone/toc

Good luck & happy trails!🍄🎣🏕