I don't have a formula or ratio that I use. But I look for function. horizontal and vertical scale and the clients personal taste when choosing a pendant. Based on the size of your island, the size of the pendants are a good size without being "too big" as you put it horizontally. How high is the ceiling? The pendant bottom should be about 36" above the island surface. So there should be enough room that when the pendant is hanging it doesn't feel crammed into the space. If you have 9' or higher you are good! I hope this makes sense...:)

Those will be too big. Depending on style, and using two, for an 8 foot island I wouldn't go larger than 10 max 12 inches.