“The theme designer in me wants to disable the UI for this altogether and present something slightly more controlled: an image size selector.”

Well, as a user, I almost never use a featured image, especially with an event listing or short post, as it often seems to dominate the page. I will likely add the image at the top left character of the first paragraph, then go to the HTML editor to see the size, and grab my trusty calculator to play with some options, keeping the ratio equal—and now I have Yellow Pencil to align the top of the image with the top of the text as I would expect to do in print!