See Javadoc
Quad defines a two dimensional four sided shape. By default the z width is always 0, where X corresponds to the width and Y corresponds to the height.
Quad q = new Quad("quad", 800, 600); rootNode.attachChild(q);
ORTHO RenderQueue the width and height will be pixel sizes. Translating along the X and Y (NOT Z) will position the quad on the screen.