Photos

Looking upwards at one of the tremendous domes which dominate the ceiling of this part of the Vatican. It is tricky getting the right angle for this sort of shot.

Workshop shows another dome which is larger than original.

Some more details:
The dome of St. Peter's rises to a total height of 136.57 metres (448.1 ft) from the floor of the basilica to the top of the external cross. It is the tallest dome in the world.[25] Its internal diameter is 41.47 metres (136.1 ft), being just slightly smaller than two of the three other huge domes that preceded it, those of the Pantheon of Ancient Rome and Florence Cathedral of the Early Renaissance. It has a greater diameter by approximately 30 feet (9.1 m) than that of the third great dome, Constantinople's Hagia Sophia church, completed in 537. It was to the domes of the Pantheon and Florence duomo that the architects of St. Peter's looked for solutions as to how to go about building what was conceived, from the outset, as the greatest dome of Christendom. Wikipedia.