mattlee- we are experiencing what appears to be a similar issue. The SVG used within the (new) card visual in desktop does not match what is published. I do no see how to change the %. I'm setting the image using a URL within the 'Image' section in Layout. There are only options to change the size of the image in pixels.
Regardless of how this works, what appears in Desktop should match what will appear in a published report, otherwise we are blind when developing in Desktop. Is this something that is actively being work on to fix, and if so, any estimate for when this might be resolved?
I posted this issue on a new thread as I hadn't seen this existing thread: https://community.fabric.microsoft.com/t5/Desktop/Card-Visual-New-Image-Size-Displays-Differently-in-Desktop-vs/m-p/4890951#M1449054