Well, it's not realy the size that matters (isn't that always the case) but rather the color and the texture of the skin and attaching vine.
Let me explain... the color of the fruit will be somehere between butter and light caramel, but more important is that the attaching vine should be shriveled and tough and the vine leaves will all probably be brown or dropped off.
If you determine it's about ready, cut the attaching vine at least two inches long and let the gourd set for a few days (preferably in the sun) to let the skin harden. This is classified as a 'winter squash', so, if you have enough to store, be sure the skin is well dried and them store in a cool, dark place.
I was tempted to say it would be ready after the meat sauce had simmered long enough, but, fortunately I resisted the urge...