I did a search on ebay for "Four Star" and saw a few unbranded models that look similar to the picture but not exactly the same. Based on the descriptions of those I'd think the wing span should tell you if it's 40 or 60 size.
If the wing span is approximately 69 inches it needs a .61 to .91 two stroke.
If it is 60 inches it needs a .40 to .52 two stroke.