Amazon maintains galleries of multiple versions and sizes of buttons that are available for your use. Button sizes depend on the category of button that you choose for your application. There are two categories of buttons: desktop and touch, with touch-style buttons being preferred. Within the desktop and touch categories, there are further options like Android, iOS, retina display, and so on.
For Amazon Pay buttons, see the Merchant Tools page on the Amazon Pay website.
For Login with Amazon buttons, see the Login with Amazon Button Guidelines page.