The scale of Hiroshima and Nagasaki makes the WTC look like a car wreck by comparison. Yet, the memorial, as Spaw notes, wasn't done on a freakishly large and maudlin scale.

I love the idea of a civilian peace garden of remembrance. Two of my favorite memorials are the Irish Famine memorial at the Custom House Quay, photo here, scroll down:

http://www.soton.ac.uk/~pg2/Fammems.html

I think some human sculptures on that scale would be appropriate too.

The other is the Garden of Remembrance sculpture of the Children of Lir being transformed into swans, seen here:

http://www.fantasyjackpalance.com/fjp/photos/city/b002/garden-of-remembrance-1.jpg

The Garden of Remembrance in North Central Dublin commemorates those who died fighting for Irish freedom. In the fountain area of the Garden is this statue by Irish artist Oisin Kelly.

The plaque reads:

"In the darkness of despair we saw a vision. We lit the light of hope and it was not extinguished. In the desert of discouragement we saw a vision. We planted the tree of valour and it blossomed.

In the winter of bondage we saw a vision. We melted the snow of lethargy and the river of resurrection flowed from it.

We sent our vision aswim like a swan on the river. The vision became a reality. Winter became summer. Bondage became freedom and this we left to you as our inheritance.

O generations of freedom remember us. The generations of the vision."

I like the idea of using birds to symbolize freedom and liberation. For the WTC, such sculptures could be used as metaphors for the strong air elements of the day, ie the airplanes, the brilliant blue sky, the choking debris cloud, the vapors of the deceased, the raining down of the millions of pieces of office paper...

I also think something like the Vietnam Memorial wouldn't exactly work in this landscape. The WTC site has a very different feel to it than the landscape surrounding the Vietnam Memorial in DC, IMO.