The 'width' and 'height' variables contain the width and height of the display window as defined in the size() function.
Original Processing.org Example: WidthHeight
// All Examples Written by Casey Reas and Ben Fry
// unless otherwise stated.
size(200, 200);
background(127);
noStroke();
for(int i=0; i<height; i+=20) {
fill(0);
rect(0, i, width, 10);
fill(255);
rect(i, 0, 10, height);
}