While in terms of the boundaries set up by Census authorities, Houston is the 5th largest city, the best way to see the size of US and Canadian metroareas is by looking at north America at night.
For example
http://www.zonu.com/fullsize-en/2009...-at-night.html
While variations in cloud cover and data processing do affect the luminosity of various Canadian and US Metropolii, and, the average densities of the metropolitan region do also, I do see a few things.
The Bay Area taken together is almost as light dense as Dallas or Houston, and covers a larger area. Baltimore-Washington and Boston are huge. The lights from numerous cities in Florida are combining into something huge, with both the Right coast collectively brighter than either Dallas or Houston, IMO. While Toronto is obviously very large (I do think that Canadian cities look like they use less lumens per/person than most US cities. The Mexican cities clearly do), Montreal is suprisingly bright.
Based upon casual analysis (adding and reducing a bit for vegetation and assuming the photos are from summer) I would say:
Boswash is still, by far the largest urban concurbation in the US.
Southern California 2nd
Chicago 3rd
Bay Area 4th
East Coast Florida 5th
To get lower down the pecking order, I suppose you have to break up the urban concurbations.
Dallas, Houston, and, Atlanta are more "stand alone" and less light networked than the top 5.
So-
New York metro (subset) 1
LA metro (subset) 2
Chicago concurbation 3
Boston metro (subset) 4
Eastern Florida concurbation 5
Washington-Baltimore combined metro (subset) 6
Bay Area (less Sack of Tomatoes, Fresno, etc) (subset) 7
Dallas metro area 8
Houston 9th
etc.
But, no doubt, Dallas, and, Houston have become huge. Like Atlanta, Phoenix, the East Coast of Florida, Denver, Charlotte, Orlando, Tampa, etc., Dallas and Houston have grown quickly at the heigth of the auto age.
As such, their future problems will be especially severe....
Toronto, fortunately, has grown with more grace than the cities just mentioned- south of the 401, that is. Seattle to Vancourver BC is no longer small, either.