how do you enlarge an i message using ios 13.1.3

You can tap on the photo to open it on the page. You can use two fingers and spread them apart to zoom the photo. See this about basic gestures. Learn basic gestures to interact with iPhone - Apple Support

Apple has options for enlarging what's on the screen.

Zoom in on the iPhone screen - Apple Support

What exactly do you mean by "enlarge"? Are you asking about making the text larger?