The Roman Empire covered Western Europe up to the river Rhine and central Germany, Britain, the Balkan Peninsula (south-eastern Europe). It also covered North Africa and part of the Middle East.
The Roman Empire covered the following modern day countries or parts of modern countries:
Western Europe: Italy, Malta, Spain, Portugal, France, Belgium, Luxembourg, Holland south of the river Rhine, southern Germany and part of central Germany, Switzerland, Lichtenstein, Austria and England and Wales.
Eastern Europe: western Hungary, part of western Slovakia, Slovenia, Serbia, Croatia, Bosnia and Herzegovina, Montenegro, Macedonia, Albania, Greece, Bulgaria, Romania, Moldova and a slither of western Ukraine.
Asia: Turkey, Cyprus, Armenia, northern Iraq, Syria, Lebanon, Israel and Palestine, Jordan, and the northern part of the coast of the Red Sea in Saudi Arabia.
Africa: Egypt, the coastal part of Libya, Tunisia, the coastal part of Algeria, and northern Morocco.
It is unlikely. The invasions of the western part of the Roman Empire by the Germanic peoples were migrations of whole peoples and were part of the migration period. This was a period of migrations from northern Europe to central and eastern Europe which had started 250 year earlier. It continued after the fall of the west with the migration of the Slavs into eastern Europe. The Slavs took over most of the European territories of the eastern part of the Roman Empire after the fall of the west (the so-called Byzantine Empire).
If it is on size then it was Alexander's empire because it was so vast that it stretched from Italy to the present India(eastawards). This ws truly very vast. If on good army power and maitenance then it would be Julius Caesar's Roman Empire(after alexander). If not, probably the Persian empire.
he would make the roman empire more powerful
No. It was the only empire to adopt the Orthodox version of Christianity, but the Roman Empire did practice Roman Catholicism at a time. Many other later European Empires also allowed Christianity, heading the crusades and converting parts of present-day Russia. Some examples of such empires would be the British Empire, the German Empire, the Spanish Empire, and the French Empire.
I can not give an exact answer but I would estimate anywhere from 30-70% of Europe, because after the Second Punic War Rome gained most of its territory.
Since the beginning of the Roman empire began with the city of Rome, you would have to say it began in the present day country of Italy.
Lots of countries in Europe did not come under the Roman Empire. Ireland would be one of them.
The City of London was a part of both the Roman Empire and modern Europe
Those three continent would be Europe, Asia, and Africa.
It is unlikely. The invasions of the western part of the Roman Empire by the Germanic peoples were migrations of whole peoples and were part of the migration period. This was a period of migrations from northern Europe to central and eastern Europe which had started 250 year earlier. It continued after the fall of the west with the migration of the Slavs into eastern Europe. The Slavs took over most of the European territories of the eastern part of the Roman Empire after the fall of the west (the so-called Byzantine Empire).
One possibility would be the formation of the Holy Roman Empire (which was unfortunately neither Holy, Roman, nor an Empire).
If it is on size then it was Alexander's empire because it was so vast that it stretched from Italy to the present India(eastawards). This ws truly very vast. If on good army power and maitenance then it would be Julius Caesar's Roman Empire(after alexander). If not, probably the Persian empire.
he would make the roman empire more powerful
First of all, I assume that you mean "Europe Outside of the Roman Empire" because the largest percentage of Roman territory was in Europe, Rome (the city and capital) is in Europe, and the over 20 current European countries were part of the Roman Empire. Europe outside of the Empire was uncivilized territory, that is to say that the people lived in Tribes that moved around the wilderness and settled in camps. They certainly interacted with each other and with the Romans and could confederate and produce armies, but there was no unifying government because there was no need (all politics could be managed locally) and it would be difficult to maintain (without easy long-distance communication, empires disintegrate).
The final war of the Roman Republic was the last of the Roman civil wars of the republic, fought between Cleopatra and Octavian. The ruling empire during those days was the Roman Empire and it would last in Western Europe until the fall of Rome in the 5th century AD.
No. It was the only empire to adopt the Orthodox version of Christianity, but the Roman Empire did practice Roman Catholicism at a time. Many other later European Empires also allowed Christianity, heading the crusades and converting parts of present-day Russia. Some examples of such empires would be the British Empire, the German Empire, the Spanish Empire, and the French Empire.
The peace of Augsburg said that principalities in the Holy Roman Empire would follow the religion of the ruler. Spain, once it had thrown the Muslims out was never part of the Holy Roman Empire, but was Catholic from that point on, and the Kings of Spain used the Inquisition to keep the protestant revolutionaries from causing the death and destruction that they had caused elsewhere. Italy was part of the Holy Roman Empire but its rulers were always Catholic.