InputField.characterLimit

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;
    }
}