Total, WTC or Hiroshima-style destruction, and 75-100% casualties, perhaps a few blocks up to 1/2 mile.
Substantial (50-75%) building losses due to fire & structural damage (i.e., quite a few buildings may have to be condemned, torn down and rebuilt), and 50-75% casualties, perhaps a 1/2-1 mile radius.
Considerable (25-50%) building losses due to fire & structural damage (i.e., extensive repairs required to most buildings), and 25-50% casualties, perhaps a 1 - 2 mile radius.
Most buildings survive with only minor damage (windows), 5-20% casualties limited mostly to those caught out in the open, perhaps a 2-3 mile radius.
Minor structural damage and small percentage of casualties (many cases of eyesight damage or loss, for example) in a 3-10 mile radius.
Some people get sick, and a small but elevated percentage eventually die due to radiation sickness, cancer, etc., downwind of ground zero.
Remember that quite a few of the buildings nearby that are not demolished by the blast will be finished off by fire. No city has a sufficiently large fire department to respond to all the fires that will be set off by a nuclear detonation anywhere within its borders. This can include wood frame houses up to a mile or more from ground zero, although the fact that with terrorists it would almost surely be a ground burst works to limit this effect.
Funny you should say this. I was just outside taking a smoke break when I looked up and saw a small single engine plane making its way from our local commuter airport. I thought to myself. Wouln't it be easy to maximize the destruction by detonating a nuclear device above the intended target using a small plane? Espescially given these terrorist fascination with airplanes.
Out of curiosity what could we expect from a .5 - 2 kiloton AIRBURST over the intended target? Could we expect an electromagnetic pulse? If so how much damage would the pulse do and how far in every direction would it travel?