About the size of your thumb nail. You dont want such a big piece that it affects your jigs feathers under water. You want that natural pulse.
^^^ And just a quick 2 cents about scents in bottles. I love em, use em, and wouldnt leave home without em. That being said, I do believe they can and will spoil over time. Specially if left in the sun. You said you've had yours for about three years now and that is probably the longest you would want to fish it. And its best to store them some place cool, and possible even in the refrigerator when not fishing.