This patch is not tested! I have some trouble to compile or run Plasma Workspace. Hope someone could help me test or give some build/test instruction. :-)
The "Digital Clock" applet uses panel height or applet height to decide the font size. If the panel is 50px high, the "Digital Clock" applet would have a huge font size around 48px. (without date or timezone) This is too big for many users.
In other desktop environment, like Unity, GNOME, Windows, macOS, similar date time applets have fixed font size that equals to the default UI font size.
This patch would add a "Font size" option to appearance configuration dialog.
If not set a font size, it uses "Default", the default font size of Plamsa. If a value was set, it will use the specific font size. It still follow the panel size and applet size limits. So the font size should never be larger than label height.