There seems to be no absolute standard yet (sigh) for setting image dimensions if https://commonmark.org/help/tutorial/08-images.html, https://www.markdownguide.org/basic-syntax/ and https://stackoverflow.com/questions/14675913/changing-image-size-in-markdown are any indication (the last link shows a lot of possibilities)... but the one that stands out to me as looking the cleanest/nicest to use is what was recently adopted by gitlab. It is also what Pandoc and a few others do, as mentioned throughout the stackoverflow thread.
Even better if the WYSIWYG GUI would allow resizing images with corner handles using that syntax.