Look at your image and ask yourself what do you notice first, the image or watermark. If all you see is the watermark, it’s too large. If you have to hunt for the watermark, it’s too small. Your goal is to strike a happy balance. Showcase your image — let the quality of your work speak for itself — and the viewer will be encouraged to look for your watermark.
Want to learn more? Check out this article, “How big should your watermark be?”