InputField.characterLimit

public int characterLimit ;

説明

入力フィールドの制限文字数。0にすると入力できる文字数は無限になります。

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;
	}
}