The paper size most used is letter-size, which is 8 1/2 by 11 inches. Also used quite often are legal-size and tabloid. Legal-size is 8 1/2 by 14 inches, and tabloid is 11 by 17 inches.
In Windows 7 - click on Start, then 'Devices and Printers. This will open a new window with all your hardware devices. Right-click the device you want to set as the default, and on the drop down menu...
If you only have one printer connected to your computer then Windows will set that as the default as well as putting a big green tick or check mark next to the printer. If you have more than one...