Paper Size	Printer Type	Courier
4″ x 6″	Thermal printer	UPS, DHL, FedEx, USPS, Royal Mail
8″ x 11.5″ (~A4)	Laser or inkjet printer	All couriers

Basically 4×6 paper size is used when printing labels with thermal printers. Unlike normal desktop printers using normal A4 papers, thermal printers are using heat-sensitive sticker paper which is faster in printing and allows you to stick the label on the package directly.