What is the symbol to change the screen resolution on a television?

See, I have sky - a digital service, and have bought a product from "one for all" and it has the sky buttons on the top. The television sometimes changes the screen size automatically how do i change it back?

My tv remote has a button at the top that says....pic size...... all i have to do is hit that and it give me a choice of the picture sizes. You also might want to go into options or setup. if all fails contact your service dealer or the tv manual