Currently you've to set the icon size manually. IMHO It could be better if the icon size would be calculated automatically depending from the plasmoid size. Implementing this feature with bug #184412 would be perfect. Consider the following usecases: 1) quicklaunch in panel: the user resize the panel, why have the user to manually specify the icon size of quicklaunch items? It should be more natural to configure how many rows of icon and leave to the plasmoid the task of calculate the dimension to fit the plasmoid size. 2) quicklaunch in desktop: the user resize the plasmoid, same like before: I've set the dimensions of the plasmoid resizing it, it has no sense to manually set the size of icons either: I've already done a resize operation, it should be enough.
As you suggested, this feature has been added with SVN commit 1121230 together with a fix for bug #184412.