I reckon you've done one of two things.
Either zoomed your web browser in so all web pages are showing large (if it's this your normal desktop wallpaper will look normal as will all the icons on the desktop) to fix this hold down the CTRL button and roll your mouse wheel back or forwards (or use + and - on the keyboard)
Or,
You have decreased the display size for everything on the computer, if you've done this then your desktop and all icons will look a lot larger. to fix this close all open windows, right click anywhere on a blank part of the background and select properties, then click on the settings tab and move the screen resolution slider to the right and click apply.