Version: 2017.4
public int characterLimit ;

Description

How many characters the input field is limited to. 0 = infinite.

using UnityEngine;
using System.Collections;
using UnityEngine.UI; // Required when Using UI elements.

public class Example : MonoBehaviour { public InputField mainInputField; public string playerName;

void Start() { //Changes the character limit in the main input field. mainInputField.characterLimit = playerName.Length; } }