I don't know what happened all of a sudden but now I need to make everything fit once again into the size of my screen, so I don't have to move the arrow right, left, up or down to see the margins of my display panel, but I don't know how. This problem also applies to my dashboard as well. Please help.

Did you change the screen resolution? That could explain your problem. Go to System Preferences>Display and select the Display tab. You can experiment from there. For informational purposes only, my 23" display has a default resolution of 1920 x 1200 and everything fits perfectly

But a changed screen resolution wouldn't make one "have to move the arrow right, left, up or down to see the margins" of one's display, would it? Seems more likely to me that spk inadvertently activated Zoom.

See Items are magnified and/or the screen moves (which you had a hand in writing, as I recall) for the solution.

Perhaps you're right. Let's see what spk says. It's certainly possible that I misinterpreted the symptom.

Sounds like you might have accidentally zoomed in by holding down the control key and scrolling the mouse.

Sounds like you've accidentally zoomed in. You can zoom out by holding down the Option and Command buttons on the keyboard and, while you hold them down, pressing the - key.