The Market place is my primary source of purchases for MSFS2020. I live in an area with low internet speeds. It would be great if providers could indicate the file size of the product upfront, before purchase, so we can plan our downloads more effectively.
This is available in Marketplace as of Sim Update 10 (1.27.10.0). It may have been available earlier.
Each product has a Required Disk Space directly under the Buy and Download button.