You want directionality and, as you're interested in tracking down a site rather than maintaining a useable (hand-held) link, probably as much of it as you can get. By the sound of it,
discretion is not an important factor for you
.
Using a cantenna, I'd go for a diameter of
circa 3.75" and a length of at least 15". The diameter is more or less a given for 802.11b frequencies and the relatively longer length gives a higher gain/narrower beam. A shorter length (e.g. 6") makes isolating sources a slower process with low-rise buildings. I haven't tried with high-rise, but I'm sure I wouldn't be able to get a good guess, externally, as to a range of floors with just 6".