How control the size of my screen

Ah, I think you enabled Zoom.

I think you accidentally hit CMD+Option+8, do it again, or open System Preferencess>Universal Access>Seeing & turn Zoom OFF. :)

Universal Access> may be Accessibility in 10.8.x

If you are talking about the browser, the size of the window should remain constant. Please describe what is happening and where it is happening more fully.