(Deep breath because I sucked at math)
Googling I found the formula:
X=r x a x Pi/180
X = the length, width, or height of the object
r = the distance to one end of the object, like the left wing tip of an aircraft pointing at you nose first.
a = angle between left and right wingtip.
Pi/180 = 3.14159../180 = .01745...
X=r x a x .01745
r x a x .01745 = X
Plug in your own values for "r" and "a".
Just a simple example, r = 1000 feet, a = 10 degrees, we get:
1000 x 10 x .01745 = 174.5 feet
How to do this based upon the image on a 2D screen, I have no idea.
But, like I said, I'm not an expert on math, and someone who is, may be laughing his butt off right now.....