You can scale the speed by the ratio of the actual resolution to the development resolution. Since the gameplay in Pong is dominated by the horizontal motion, you can use the ratio of the widths:
ballSpeed = int(ballSpeed * width / 640.0);
You will probably want to do the same to the size of all the screen elements too, such as the ball.
This all assumes that you define "speed" as the time taken to cross the entire screen, not to cover a given number of inches. That calculation would depend on more than just the resolution.