Small Unity3D editor script that will create a BoxCollider that encapsulates all Renderers in the selected Transform's hierarchy.

FittedBoxCollider.cs

using UnityEngine;
using UnityEditor;
using System.Collections;

public class FittedBoxCollider {
	[MenuItem("GameObject/Create Other/Create Fitted BoxCollider")]
	static void FittedBoxColliderMenu() {
		Transform transform = Selection.activeTransform;
		Quaternion rotation = transform.rotation;
		transform.rotation = Quaternion.identity;

		BoxCollider collider = transform.collider as BoxCollider;
		
		if (collider == null) {
			transform.gameObject.AddComponent<BoxCollider>();
			collider = transform.collider as BoxCollider;
		}

		Bounds bounds = new Bounds(transform.position, Vector3.zero);

		ExtendBounds(transform, ref bounds);
		
		collider.center = bounds.center-transform.position;
		collider.size = bounds.size;
		
		transform.rotation = rotation;
	}
	
	[MenuItem("GameObject/Create Other/Create Fitted BoxCollider", true)]
	static bool ValidateFittedBoxColliderMenu() {
		return Selection.activeTransform != null;
	}
	
	static void ExtendBounds(Transform t, ref Bounds b) {
		Renderer rend = t.renderer;
		if (rend != null) {
			b.Encapsulate(rend.bounds.min);
			b.Encapsulate(rend.bounds.max);
		}
		
		foreach(Transform t2 in t) {
			ExtendBounds(t2, ref b);
		}
	}
}