Textwrap.cs

using UnityEngine;
using System.Collections;

public class Textwrap : MonoBehaviour {

    public string theText = "Hello world blah-dy-blah!";
    public float theWidth = 2f;

    private TextMesh texty;

    void Start () {
        texty = GetComponent<TextMesh>();
    }

    void Update () {

        texty.text = "";
        string[] words = theText.Split(' ');
        for (int i = 0; i < words.Length; i++){

            string workingString = texty.text;
            texty.text = workingString + " " + words[i];
            if (i==0) texty.text = workingString + words[i];
            if (texty.renderer.bounds.size.x > theWidth && i > 0) texty.text = workingString + "\n" + words[i];
        }
    }
}