Can I Specify the System Disk Capacity When I Create an ECS Using an Image?

Yes, but you cannot create a system disk smaller than the original and the maximum allowed is 1,024 GB.

When you use a full-ECS image to create an ECS, the system and data disk information defaulted by the image will be automatically displayed. You can increase the capacity of a system disk or data disks, but cannot decrease it.

For details, see Creating an ECS from an Image.