Not sure exactly what you have done wrong but if you try hitting the Ctrl button (bottom left on keyboard across from space bar) and + button (top right hand section of keyboard, on numberic section of keyboard) at the same time. You may need to press them a few times at the same time to bring the screen back down to normal size.
P.S. Just edited this post to show + and not minus. Misunderstood and thought screen was too big but see now it was the opposite and was too small.