Subject: How do you get the diameter of an asteroid or irregular shape?

I have not been able to find a way of getting the measure of a diameter for an 
asteroid since this is a three dimensional an irregular figure I have gone as 
far as finding the diagonal and volume but yet every time I research about 
another fact on asteroids I find all measurements to given in diameters, If you 
could explain to me what formula to use or how is done it would be of great 
help, I thank you in advance for all feed back you could provide me with. 

Re: How do you get the diameter of an asteroid or irregular shape?