The specific types of label you should use

If you are using a laser of inkjet printer, you should use 5.5″ × 8.5″ shipping labels, such as the Avery White 5-1/2 × 8-1/2-Inch Ink Jet Printer Labels

You might receive them for free from various shipping carriers, or you might find them at your local office supply.

However, if you do a lot of shipping, we recommend purchasing a thermal printer capable of printing 4″ wide labels such as the Zebra LP 2844. This will save you money over the longterm since thermal printers do not require ink.