[dgatwood]

For whole-screen content, I tend to use a separate HTML page and SVG. It is pretty much the only approach I'm aware of that will make things work robustly (almost) everywhere.

For smaller images, I'd normally expect them to be shown at actual size, rather than specifying size information, and I normally pick the image size to be something reasonable on an average device screen, and just let it be small on other devices. Whether that works or not for you depends on the layout.

But if you want to do something more complex than that, then yes, you'd pretty much have to have classes for each image size.