Is it possible to define the default Media Width using theme.json?

I’m not seeing anything in the documentation and I’ve tried a few things in my theme.json but I haven’t been able to set the default media width field to anything other than 50. I have a site I’m building where it’d be really nice to set the default Media Width to 60 so the user doesn’t have to change the field every time they add a Media Text block to the page.

In the “settings” block my theme.json code, I’ve tried the following with no success:

"core/media-text": {
                "custom": {
                    "mediaWidth": 60
                }
            },

"core/media-text": {
                "attributes": {
                    "mediaWidth": 60
                }
            },

Is this just not possible or what am I doing wrong?