Markdown is quite different from MS Word. Whereas a Word document contains layout, presentation, and content information, Markdown is only used to structure content. This gives Markdown documents the advantage of being independent from how they are presented. Different applications can present the contents of the document in different ways - for example, one application could render the document as a single wide column, another in newspaper style columns. Different layouts may require images within the document to be rendered in different sizes too - for example, a mobile layout will typically require that a smaller version of the image is displayed compared to the layout that is rendered when using a desktop browser.
I think that image dimensions are a presentational concern that do not belong in Markdown.
That said, for power users who want to explictly define additional attributes such as image height and width, we could just use consistent attribute syntax rather than inventing a new syntax specifically for image dimensions.