GUILayout.ExpandWidth

설명

Option passed to a control to allow or disallow horizontal expansion.

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();
    }
}