Looking at your image it looks simply like overexposure to me, you can also see that the streetlights appear bigger. Also the lights in the windows are a lot brighter than normal.
If this was just an exemplary image, I would suggest to look for an image with "normal" exposure. If not it may be possible to measure/calculate the amount of overexposure in this picture, and go on from there. I never tried that though, so I sadly can't give you any advice on how to proceed with calculating the overexposure.
Taking into account that your goal is to calculate the spot-size of a light-emitting object, I don't think you can apply exactly the same methods as for calculating "normal" objects. Reason is, that leds and lasers for example emitt light in a cone and not a straight line. So the further you are away from the source, the bigger the diameter of your lightbeam gets (taking out of consideration for a second that the intensity goes down). In the end this also has a strong correlation to the amount of exposure in the picture you take.
In conclusion, I would suggest you try varying the amount of exposure to a point where the lightbeam becomes round and gets a clear edge. From there you would have to calculate/measure the distance from your camera to the light-source and find out the emission angle of your light-source(or assume, some led's emmit light with an angle of 60° for example). Then you could calculate the size.
I hope I was able to display, that this process is much harder for light emitting objects than "normal" one's and how many additional things you would have to consider to make a precise calculation.
Note that everything I said before didn't even consider the presence of a lense.