The size parameter should accept relative values, like "100%". When including large images, I would like it to be scaled relative to the available space.
100% of what?
The purpose of
<span class="createlink"><a href="/ikiwiki.cgi?do=create&from=todo%2Fimg_tag_should_support_relative_size&page=page" rel="nofollow">?</a>page</span>is to scale large images, for example photos, down to a more web-suitable size. When ikiwiki rebuilds the website, it cannot know how large visitors' web browser windows are going to be, so it cannot scale the image relative to the size of a visitor's web browser window.
The closest thing it could do would be to not scale the image at all (potentially a very large download if it's a high-resolution photo), and use CSS or
<img sizes=...>to ask the visitor's web browser to scale the image relative to something the web browser knows, such as the viewport size.
<img sizes="..." srcset="...">, it would be possible to extend
<span class="createlink"><a href="/ikiwiki.cgi?do=create&from=todo%2Fimg_tag_should_support_relative_size&page=page" rel="nofollow">?</a>page</span>to produce more than one resized image and let the visitor's browser choose which one to download, but I'm not sure what a good syntax for that would look like...
"The available space" is not something we can use, because current HTML standards do not offer that. In HTML5 it is possible to base sizes on the viewport (window) size, but the available space (excluding sidebars etc.) is not something the browser can know in advance, because it needs to know how large images are before it carries out layout calculations, and it needs to carry out layout calculations before it can know the available space.