I think there are instances where both would want to be able to control the size.
In your example I'd agree that the gallery should be controlled by the designer, but the map (inline content) should be controlled by the content creator.
I do think shortcodes could work well here, but it could also work in a different (simplier) way.
What if instead of
gallery = ["image1.jpg", "image2.jpg", "image3.jpg"]
The editor refered to these same files as
gallery = ["image1.50%.jpg", "image2.600x600.jpg", "image3.480.jpg"]
It's a bit of a hack and it's just an idea (we don't have any interface for this yet), but I like it.
I think it would be pretty powerful and it could work well in content and meta-data.
The idea is that three formats would be supported (though there's probably more).
- Percent (int + "%")
- Resize and crop if needed (H int "x" W int)
- Resize (int)
For the resize one the int would likely represent the longest size.