How to define the size or proportion of images inserted into markdown?

Defining the size or proportion of images in Markdown requires understanding that standard Markdown syntax, as originally specified, does not include native attributes for controlling image dimensions. The core syntax `` is deliberately simple and only handles the insertion and alternative text of an image. Therefore, any sizing must be achieved by extending the basic syntax through the integration of raw HTML or by leveraging the specific features of the Markdown processor or the surrounding publishing platform being used. This fundamental constraint means there is no single, universal Markdown method; the approach is entirely dependent on the context in which the Markdown is rendered.

The most reliable and widely supported method is to use inline HTML within the Markdown document. Since Markdown permits the inclusion of HTML tags, one can replace or supplement the standard Markdown image syntax with an HTML `<img>` tag, which includes `width` and `height` attributes. For instance, `<img src="image.jpg" alt="alt text" width="500" height="300" />` provides explicit pixel dimensions. For proportional scaling, typically only one attribute is specified, or CSS styling is applied inline, such as `style="width: 50%;"` to set the image to 50% of its containing element's width. This method offers precise control and is the de facto standard for achieving image sizing in contexts where pure Markdown falls short, such as in static site generators like Jekyll or Hugo.

However, numerous Markdown implementations and platform-specific flavors have introduced their own proprietary syntax extensions to address this limitation without resorting to HTML. For example, some variants, like that used in certain documentation generators, allow appending attributes within curly braces or using a trailing syntax like `{: width="100px"}`. Popular platforms like GitHub Flavored Markdown do not support such extensions for images in their core rendering, but they may allow relative sizing within their own UI contexts, like in GitHub Wikis or README files, through HTML. Conversely, tools like Pandoc or Markdown processors within content management systems often provide their own extension mechanisms, which must be consulted in their respective documentation. The mechanism is therefore not about Markdown itself but about the rendering engine's feature set.

The primary implication is that workflow and portability must be carefully considered. Relying on raw HTML ensures maximum compatibility across any processor that allows HTML, which is most of them, but it breaks the purity of plain-text Markdown and may not be supported in highly sanitized environments that strip HTML for security. Depending on a platform's proprietary syntax locks the document into that ecosystem, reducing its utility as a portable plain-text format. The optimal practice is to define images using the standard Markdown syntax for broad readability and then add sizing instructions via the most minimally invasive method supported by the target rendering pipeline, whether that is inline HTML for a personal blog or a specific extension for a corporate documentation system. This maintains the document's structural integrity while achieving the necessary presentation control.