let maxDiam = 200;

​

function setup() {

  createCanvas(windowWidth, windowHeight);

}

​

function draw() {

  background(220, 20, 120);

  let mDiamInv = (mouseY / height) * (maxDiam - minDiam) + minDiam;

  let mDiam = maxDiam - mDiamInv + minDiam;

  ellipse(mouseX, mouseY, mDiam);

}

​