I don’t think that’s a good idea. Firstly, I guess that these dimensionless values are meant to be in pixels. Which is not a sensible choice for sizes nowadays: while 100 pixels are a third of the screen width on an iPhone, they’re only about an eighth on an ipad and even less on the desktop. Why wouldn’t I want to see the image larger on a larger screen?
Secondly, such a modification breaks compatibility. Markdown is already fractured enough as it is. Introducing another “feature” is not helping.
Thirdly, the solution is already there, and it is called CSS.
width=“xxxx” … sets the size with xxxx pixels style=“float:yyyy” … sets the float as “center, right, left” … cent is text above and below, right text is left of image, left text is right of image.