We've got instructions on how to set up a dev environment here on docs.mattermost.com.
And I was actually wrong in my previous post. You shouldn't actually need to touch the markdown parser to do this. Markdown actually includes things after a space as part of the URL for some reason, so if you post
in a message, the image method in webapp/utils/markdown.jsx is called with the first argument as "https://example.com/avatar/user.png =50x50" to generate the HTML. You can just change it to look for the size at the end of the href. If you added that and then some unit tests to the webapp/tests/ directory, that'd be amazing.