This doesn't appear to be possible, but best place to look
would be here.
Although I have not tried this, if you are talking about a desktop widget, it is possible by configuring size of icons on the desktop also affects widgets. In System Settings/Appearance/Icons there is a "Configure icon sizes" button where you select which icons, (Desktop, Toolbar, Main Toolbar, Small icons, Panel, Dialog) and use the slide control to adjust the size.
