I've added some randomness to the initial viables, so if you refresh the page it'll be a little different. :)
var rand = Math.random,
x = 100,
y = 100,
xSpeed = rand() * 4,
ySpeed = rand() * 4,
radius = rand() * 28 + 6,
diameter = radius * 2;
function setup() {
createCanvas(500, 200).parent('canvas-container');
}
function draw() {
background(255);
x = x + xSpeed;
y = y + ySpeed;
if ((x > width - radius) || (x < radius)) {
xSpeed = xSpeed * -1;
}
if ((y > height - radius) || (y < radius)) {
ySpeed = ySpeed * -1;
}
stroke(0);
fill(175);
ellipse(x, y, diameter, diameter);
}