How can we give you dimensions, if you don't know the size of your fingers? Such finger tabs, as I like to call them, come in various sizes. Small for kids, medium for small hands and women, large for most men, XL for large hands...you get the point.
Copy that picture into MS Word and enlarge it some. Print several sizes, each slightly enlarged. Cut them from the paper and test fit, to see which size fits your hand best. Then trace to leather and cut.
