To secure the intellectual property being exchanged between Connect and your application, we require that download requests be made over HTTPS to prevent download URLs from being sent in the clear and potentially intercepted. The Connect API's CreateSession operation will provide you with both a secure and standard token for use in HTTPS or HTTP requests respectively.
Download size codes (a.k.a. SizeKey) are returned in the GetImageDetails response body under SizesDownloadableImages. In addition to the SizeKey, each downloadable image's DPI, file size, height and width (in pixels and inches) is returned in this structure.