The building of the Cathedral or Duomo began on Sept. 8, 1296 and went on until 1375. Building of the dome had to wait until 1420, the year in which Brunelleschi won the competition for doing the work. Building of the dome lasted till 1434. The lantern (the structure on top of the dome) was started in 1445 and was finished in 1461 with the gilded sphere with cross on top of it.
The dome at the drum is 45.52 meters in diameter and 91 meters high at the upper drum. With the lantern on top of it, the height goes to 107 meters.
For more information on the Cathedral, please click the link --> http://en.wikipedia.org/wiki/Florence_Cathedral