How to attach a wall plate to an old junction box with different screw hole spacing?
What you have there is a 3-1/2" round junction box intended for lamps.  That is a standard size, though largely obsolete today in favor of 4" round.  

A 3-1/2" round junction box cover should do the job.  Most well equipped hardware stores and every electrical supply will sell it.  

You might also find decorator covers but look for ones that will work with 3-1/2" round boxes.