Changing image size in Markdown

You could just use some HTML in your Markdown:

Or via style attribute (not supported by GitHub)

Or you could use a custom CSS file as described in this answer on Markdown and image alignment

CSS in another file:
img[alt=drawing] { width: 200px; }

With certain Markdown implementations (including Mou and Marked 2 (only macOS)) you can append =WIDTHxHEIGHT after the URL of the graphic file to resize the image. Do not forget the space before the =.

You can skip the HEIGHT