The current size of the showdesktop/minimzeall widget is much larger than it needs to be. This patch reduces the maximum size in panels to smallMedium (22px).
This also needs to be applied to the minimizeall widget, as this is the directly (right click) selectable alternative to showdesktop widget, so the icon size change would revert if a user switched the panel widget to this.
While I'm all for making the show desktop icon stick out less I don't think randomly limiting the size of one particular applet is a good approach. The iconSizeHints is there for a reason, defining the maximum size of icons in the panel.
IIRC you have to make it use a PlasmaComponents SVGItem instead of an IconItem (but a Plasma person could confirm)
You're not necessarily wrong, but I do worry that we'll get bug reports where people say, "The Show Desktop item has a small vertically centered icon when the panel is tall"
I couldn't figure out how to make the widget get the icon from the desktop theme
Plasma IconItem primarily uses icons from Plasma theme, if such exists, or told so otherwise by setting usesPlasmaTheme to false
@ndavis in the future, the best way to do this would be to land it on the Plasma/5.16 branch, then merge that branch to master. This process is detailed at https://community.kde.org/Infrastructure/Phabricator#Landing_on_the_.22Stable_branch.22