Why is my computer desktop so large and everything that i click on ontop of my desktop?

When i turned on my computer today i noticed that my desktop icons are really big and when i get on the internet its really big, like the words and pictures. My itunes are doing the same thing. I tried to play my sims 3 game and it wont let me because of my video graphics being too big. How do i fix this problem? Even the bottom of my desktop that shows what icons i have on and where the date and time is, is big too. Please help me fix this problem.