what`s the point of that?
Dropping it down will make XBMC bigger. Your 4K video rendering will not be affected by doing this. So you will still be able to see 4K transmissions (what few there are).
Until developers either produce separate plugins for different resolutions or include code that makes them adapt I'm afraid we're stuck with plugins looking undersized even on HD boxes. Sadly, we here, have no control over this whatsoever.
Still say, if you want it sensible size, your best bet is to dump the default skin for Metrix (non-HD). Again this will not affect your enjoyment of 4K media content which will still display awesomely.