int horizon = 300;

void setup() {
  size(600,500);
}

void draw() {
  background(204);
  line(0,300,width,300);
  float diameter;
  if (mouseY < horizon) {
    // we want interpolation mapping horizon --> 0 and topEdge --> 100
    diameter = map(mouseY, horizon, 0, 0, 100);
  } else {
    // we want interpolation mapping horizon --> 0 and bottomEdge --> 100
    diameter = map(mouseY, horizon, height, 0, 100);    
  }
  ellipse(mouseX, mouseY, diameter, diameter);
}