Sounds like a linear equation. If the ball sprite is 100 x 100 (when 1:1), then, it should be something like:
Every tick -> ball.width = 100 * [here a function with ball.X (screen center) wich gives as a result 1, so it multiplies 100 by 1 to get the max size of the ball], when at the center of the screen.
And at the left edge, where ball.X = 12,5, the same equation should give you something like:
Every tick -> ball.width = 100 * [here the same function on ball.X (12.5 for left edge) wich gives as a result 0.25, so it multiplies the 100 by 0.25 to get your 25 pixels wide ball]
Also, I guess Every tick -> ball.height = ball.width after the above.
Sorry I can't give you the equation, but it should be easy to google by "finding linear equation with 2 points", those points being [screenwidth/2, 1] and [12.5, 0.25].