It is difficult to answer. are you asking the biggest in size or in population and if in population it depends on the relative world population at that time. Any how, refer to links belowfor more information.
There is no Islamic Empire.
In 1492 the Islamic Empire finally left Spain but i dont know when it began.
The Abbasids, as the ones from the Islamic Empire.
The biggest amphitheater in the Roman empire was the Colosseum.
The Mongol Empire
There is no Islamic Empire.
The title of the Islamic Empire was Caliphate. The Caliph was called Amir-ul-Momineen.
It depends on the Islamic Empire in question. Assuming that you are talking about the Abbassid Caliphate (which is most commonly referred to as "the Islamic Empire"), the group that brought them down was the MONGOLS.
In 1492 the Islamic Empire finally left Spain but i dont know when it began.
Yes. The Mughal Empire was a great Empire of the Muslims in India.
Sunnis is the biggest Islamic school. The other one is Shiites. refer to related question for more information.
Ottoman Empire
Different in different parts of the empire.
The Abbasids, as the ones from the Islamic Empire.
The biggest amphitheater in the Roman empire was the Colosseum.
The Colosseum at Rome was the biggest amphitheater in the empire.
Rome was the biggest city in the Roman Empire.