How to change text size in c# by user?

T$$anonymous$$s is one way to do it, just assign the text object to the script in the editor and set the value you want to add or subtract in the range variable. Then assign the methods ClickPlusButton and ClickMinusButton at the buttons you want to use.

 using System.Collections;
 using System.Collections.Generic;
 using TMPro;
 using UnityEngine;
 
 public class TestScript : MonoBehaviour
 {
 
     public TextMeshProUGUI theText;
     public float range = 1f;
 
     public void ClickPlusButton()
     {
         theText.fontSize+=range;
     }
 
     public void ClickMinusButton()
     {
         theText.fontSize -= range;
     }
 
 }