// Draws a  button with a  fixed width
function OnGUI() {
    GUILayout.Button("A Button with fixed width", GUILayout.Width(300));
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void OnGUI() {
        GUILayout.Button("A Button with fixed width", GUILayout.Width(300));
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def OnGUI():
        GUILayout.Button('A Button with fixed width', GUILayout.Width(300))