how do I make keypad keys bigger on iphone 13 pro?

It’s not possible. They are already as large as they can feasibly be…

This article may help you use the swipe to type feature introduced a few years ago in iOS, which may be something that could help you: https://appleinsider.com/articles/21/03/09/how-to-use-slide-to-type-on-iphone

To my knowledge it is not possible.