eBay/Paypal Shipping Labels

For normal letters, most people will use Standard Address Labels which measure 1-1/8" x 3-1/2" (28x89mm). These labels will print 3-4 lines of clearly legible text plus the USPS official barcode.
As an alternative, some people opt for the Large Address Labels which are a little taller than the standard address. The Large Address measure 1-4/10" x 3-1/2" (36x89mm).
For larger packages, most people choose the Large Shipping Labels. Measuring 2-5/16" x 4" (60x101mm) these are big enough to print a full size address, with the USPS barcode and you can also fit your Return Address on there too.
Speaking of your return address, a very popular way to clearly mark the origin of your mail is to place a Return Address Label on the back of your envelope/box. These measure 3/4" x 2" (19x51mm) and most people will just print off a small qty like 20 or 30 and have them on standy ready to place on their mail.