GUILayout.ExpandWidth

説明

コントロールの水平方向の拡張を許可/禁止するオプション

If this is true, the enclosed UI elements can expand to fill the available horizontal width.

.

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour
{
    void OnGUI()
    {
        GUILayout.BeginVertical();
        GUILayout.Button("Short Button", GUILayout.ExpandWidth(false));
        GUILayout.Button("Very very long Button");
        GUILayout.EndVertical();
    }
}