screen bigger than monitor......

Few days ago, the screen of my mac became bigger than my monitor so i see the screen partually and when I move the mouse I see the rest of the screen......I tried already to change the resolution of the monitor (the best for it is 1024x768) but it didn't changed anything......I tried also to change Hz and the colour but nothing

the strange thing is that when I start the Mac Mini, at the beginning, for a moment the screen returns normal but after 2 seconds it changes and come bigger

thanks for the one that can help me