Doesn't mean it is a bunch of little green/gray guys - but it is a UFO (by definition.)
I don't know what it is, or isn't. But you can't tell the size unless you know how far away it was, compared to the aircraft's distance. The "inset" is a blowup of part of the picture, you can tell by the difference in the background texture. The picture on the linked website shows the actual "size" of whatever it is, relative to the aircraft somewhat more clearly, but one still can't tell relative distances. Many missles don't have exhaust trails for much of their flight, they boost and then coast. They are often "super elevated", that is aimed way over the target, so they gain potential energy while the motor is burning, an then "glide" down to the target.