It look like it my be that by way of picture, but the dark "bulb" is approximately 3/4" (19mm). I can take a picture of the other side in a bit...but in the interim, it only has factory 3M two-sided tape on it. I guess this "bulb" is meant to be struck to something, likely a vertical surface I am guessing.
It looks like it might be an IR emitter, perhaps for something like a Logitech Harmony or other media controller device? They usually have 2.5mm or 3.5mm jacks as connectors and the front of the housing will have an IR LED in it.