Using the “modern” $transclude widget parameters available in TW5.3.0 and above (released 01-July-2023) , you can write something like this:
@@fill:blue;<$transclude $tiddler={{!!icon}} size=“10em”/>@@
Note that the $tiddler parameter has a leading $ to enable “modern” parameter handling, but the size parameter does NOT have a leading $, indicating that it is an extra parameter that will be passed to the icon tiddler for use as a variable within the SVG content definition.
You will need to upgrade from TW 5.1.24-prerelease (see https://tiddlywiki.com/upgrade) to get this new parameter handling.
Also, please note that, while this Google Group is still available, it is no longer the primary location for posting questions and discussions about TiddlyWiki and the TiddlyWiki community has moved to https://talk.tiddlywiki.org where you will find much more activity and timely responses.
enjoy,
-e