About the size of the aircraft from the thread: "Why the towers collapsed: hit at vulnerable point" @ URL http://www.freerepublic.com/forum/a3b9e9dd8152e.htm (sorry, not up to speed on posting working links in html at the moment). The 757 has a 124-foot wingspan, is 155 feet long and can weigh 100 tons. A 767 is bigger, with a 156-foot wingspan and 159-foot length and can weigh a maximum of 200 tons. (A 747 is more than 200 feet long and can weigh 400 tons.)
A couple of notes. It has come to my attention that UA Fl # 175 may have been a 757. However, I would not rule out that this stems from earlier reports that both towers were struck by American Airlines planes, in which one was a 757 and one was a 767. Also, the time should read EDT, not EST.