@AE Thanks for reaching out. In standard Markdown, you cannot directly set the size of an image, meaning that this limitation comes from markdown, not developer portal.
To correctly set height and width for an image you need to use the below html code.
<img src="<source>" width="500" height="250">
do let me know incase of further queries, I would be happy to assist you.
Please accept as Yes if the answer provided is useful, so that you can help others in the community looking for remediation for similar issues.