Is there a reason there is no spec for specifying image dimensions? It seems fairly common.

Is this something plugins are going to need to take care of?

Agreed this would be nice to have provided it can be done in a way that does not add a bunch of rules or complexity.

There's a discussion about this on the pandoc issues page where I proposed the following syntax. (However, image dimensions should be seen in light of the issue of a generic attribute syntax.)

The syntax for inline images is:

![caption](image.png){#id .class key1="val 1" key2="val 2"}

and for reference images:

![caption]
[caption]: image.png {#id .class key1="val 1" key2="val 2"}

• For HTML/EPUB all attributes are passed through “as is”. This provides elegant support for all future responsive image properties (e.g. srcset, sizes) as well as a consistent way to specify the plain old HTML attributes width and height. So if you write ![](image.png){width=300 height=200}, you’ll get <img src="image.png" width="300" height="200">. (This is also useful to instruct the browser to reserve a 300x200 pixel box until the image is loaded to prevent a jarring reflow when the image arrives, but this could also be done with CSS.)
• The other writers ignore attributes that are not supported by their output format.
• The width and height attributes need special treatment. When used without a unit, the number’s unit is in pixels (to keep the syntax consistent with passing through HTML attributes). However, one of the following units can be used: px, cm, inch and %.
  • Pixels are converted to cm/inch for output in page-based formats like LaTeX and vice versa for output in HTML. Pandoc could introduce a --dpi option, but the default would be 96dpi.
  • If % is used, the result would be for example <img href="file.jpg" style="width: 50%;" /> or \includegraphics[width=0.5\textwidth]{file.jpg} (LaTeX) and finally \externalfigure[file.jpg][width=0.5\textwidth] (ConTeXt).
• In formats like HTML and ConTeXt that have a notion of a class, ![](file.jpg){.myClass} will result in <img href="file.jpg" class="myClass" /> and \externalfigure[file.jpg][myClass] respectively. (Note that the complete ConTeXt syntax is \externalfigure[file.jpg][width=8cm, height=1cm][myClass] and the class has always lower precedence, whereas CSS has always higher precedence than the HTML attributes.)
• When no attributes are specified, the behaviour is obviously the same as now (DPI image metadata, for example, can still be used).

I get this, and the appeal for a generic solution.

But there is also an appeal for a very simple solution here: ![caption](image.png 100x100) its very nice on the eyes.