Along with the new ability to set icon style from CSS, it would be incredible useful to be able to specify icon size as well. Specifically, this would be useful for making elementary and GNOME apps fit in better when ran on each other's environments especially regarding toolbar/headerbar icon size and style.

I would assume the css would look something like this (for example):

.toolbar GtkImage {
    height: 24px;
    width: 24px;
}