152 page O'Hare Report pdf here, from the National Aviation Center on Anomalous Phenomena
http://www.narcap.org/reports/010/TR10_Case_18a.pdf Witness A said it was equivalent in angular size to a quarter held at arm's length or about 2 deg. 4 min arc. The object would have to be 88 feet across at 1,900 feet altitude to have the same angular size. Similarly, witness D said the UAP was about the same size as the end of a pencil held at arm's length (about 36 minutes of arc in diameter) which is equivalent to a 22 foot diameter object located 2,093 feet away.22 If the UAP were at a lower altitude then its visual size would increase slightly. Thus, if it is assumed that the UAP was at 1,500 feet altitude these same two witness's estimates become 69 feet and 18 feet, respectively. Witness J.H. standing in a parking lot about a mile east (see Appendix E <4>) estimated its diameter to be from 25 to 30 feet. Witness 'Rampagent X' located somewhere near concourse C thought it was about twenty feet diameter. These are remarkably similar estimates.
70 feet, 18 feet, 100 yards?
Also from your link:
Phone Conversation Between United Ramp Tower (R)
and FAA Area Supervisor (T) in O'Hare Tower
T "Go ahead, tower, Dwight"
R "Dwight?"
T "Yes"
R "What happened to Dave? Did he have to take a break because I called him?"
T "No"
R "This is Sue from United" (laughter)
T "Yes" (serious tone)
R (12 sec. pause) "There was a disc out there flying around"
T "There was a what?"
R "A disc"
T "A disc?"
R "Yeh"
T "Can you hang on one second?"
R "Sure"
--------------------
(33 sec. pause) Contains ramp tower background chatter "He's working traffic",
"OK, I'll be right back" "Alright"
---------------------
T "OK, I'm sorry what can I do for you?"
R "I'm sorry, there was, I told Dave, there was a disc flying outside above Charley 17
and he thought I was pretty much high. But, um, I'm not high and I'm not
drinking."
T "Yeh"
R "So, someone got a picture of it. So if you guys see it out there …."
T "A disc, like a Frisbee?"
R "Like a UFO type thing"
T "Yeh, OK."
R "He got a picture of it." (laughs)
T "How, how, how high above Charley 17?"
R "Well, it was above our tower. So"
T "Yeh"
R "So, if you happen to see anything (she continues to laugh)"
T "You know, I'll keep a peeled eye for that"
R "OK"
T "Bye"
Haines and his co-authors conclude, in part:
This investigation has determined that, based upon the testimony of multiple eye witnesses in different locations at O’Hare International Airport, a small physical, and apparently solid object hovered over the United Airlines concourse area for at least ten minutes or more but was not detected either by radar or visually by air traffic controllers in the tower. For this reason the object is considered a definite potential threat to flight operations at O’Hare.