It is all to do with your screen resolution.
The Windows display is measured in pixels (very small dots), a number of pixels wide and a number of pixels high, 800 x 600 for example.
Obviosuly the MORE pixels that are shown the SMALLER each item will be on the screen.
To see what resolution you are currently on RIGHT click anywhere on the desktop, and chose Properties, then on the window choose the Settings tab.
You will see a Screen Resolution area showing what your current pixel setting is.
On a modern PC it may be 1024 x 768 or even 1280 x 1024. Write down your current seting in case you want to go back to it.
Now change the setting so it is LOWER (smaller numbers), so if it is currently 1024 x 768 make it 800 x 600 and press OK.
If you are happy with that leave it as it is, if not play around with the settings until you are happy with it.
There tends to be standard settings, which are
640 x 480 (bigger icons and windows)
800 x 600
1024 x 768
1280 x 1024 (smaller icons and windows)