The customer may observe after going to SD Cloud SRX/Device Management/Software Images the Size column show only a - instead of the image size

This is expected behavior, there are two ways the images are uploaded:

Add an image

Stage an image

When you add a software image, only details such as the URL, the checksum details, and the properties are stored on Juniper Security Director Cloud The actual image is uploaded only when you stage the image.

For more details on uploading images please see the documentation in the below link:

https://www.juniper.net/documentation/us/en/software/sd-cloud/sd-cloud-user-guide/user-guide/topics/concept/about-images.html