Dimensions available for a gang toggle light switch cover
From you measurements, what you have is a single gang box.  If you have a duplex switch (two switches stacked on top of each other), then those use the same box cover as a duplex outlet:

 

If for some reason, a duplex cover does not fit, you must have a very old duplex switch style that isn't made anymore.  In this case, the easiest thing to do is to replace the switch and get a standard cover for it.