There are no set rules but around 1:3 coffee to water/ice is how you will typically get an Iced Americano served in coffee houses like Starbucks. We definitely recommend experimenting with different volumes to find your perfect blend.
If you like your caffeine hit strong, go for two or three shots of espresso and less water/ice, and to make it weak, tune your ratio to 1:5 coffee to water/ice. Don’t forget the ice will melt if left long enough, which will further dilute the flavor of your drink.