float x = 400, y=100;
int a=20,b=10;
void setup() { size(800, 200); }
void draw() {
rect(x-10, y-5, a, b);
}
void keyPressed() {
  int k=' ';
if (keyCode == RIGHT) {
x++;
} else if (keyCode == LEFT) {
x--;
} else if (keyCode == UP){
  y--;
} else if (keyCode == DOWN){
  y++;
}
}

float x = 400, y = 100;
int a = 20, b = 10;
boolean isExtended = false;
void setup() { size(800, 200); }
void draw() {
  fill(255);
  rect(0, 0, 800, 200);
  rect(x - a / 2, y - b / 2, a, b);
}
void keyPressed() {
  if (keyCode == RIGHT) {
    x++;
  } else if (keyCode == LEFT) {
    x--;
  } else if (keyCode == UP){
    y--;
  } else if (keyCode == DOWN){
    y++;
  } else if (keyCode == ' ') {
    isExtended = !isExtended;
    a = isExtended ? 40 : 20;
    b = isExtended ? 20 : 10;
  }
}