156 meters 156 meters(510 ft / 528 Roman feet)
The Roman Colosseum or Coliseum, originally known as the Flavian Amphitheatre, was commisioned in AD 72 by Emperor Vespasian. It was completed by his son, Titus, in 80, with later improvements by Domitian. The Colosseum is located just east of the Roman Forum and was built to a practical design, with its 80 arched entrances allowing easy access to 55,000 spectators, who were seated according to rank. The Coliseum is huge, an ellipse 188m long and 156 wide.  All citizens could easily be kept in the Colosseum because of its size. The Colosseum was a massive structure. It covered over six acres of land, from end to end the Colosseum was 620 feet long, 513 feet wide, and over 160 feet tall. To fill and empty such a large building very quickly would be difficult. The architects came up with a system of arches and staircases inside the building. The Colosseum is situated just east of the Roman Forum. Construction began under the emperor Vespasian in 72 AD, and was completed in 80 AD under his successor and heir Titus. Further modifications were made during the reign of Domitian (81–96). Construction of the Colosseum began under the rule of Vespasian in around 70–72 AD (73-75 AD according to some sources) The Colosseum had been completed up to the third story by the time of Vespasian's death in 79. The top level was finished by his son, Titus, in 80, and the inaugural games where held in A.D. 80 or 81. The Colosseum could hold, it is estimated, between 50,000 and 80,000 spectators, having an average audience of some 65,000; it was used for gladiatorial contests and public spectacles such as mock sea battles, animal hunts, executions, re-enactments of famous battles, and dramas based on Classical mythology. Construction of the Colosseum began under the rule of Vespasian in around 70–72 AD (73-75 AD according to some sources) The Colosseum had been completed up to the third story by the time of Vespasian's death in 79. The top level was finished by his son, Titus, in 80, and the inaugural games where held in A.D. 80 or 81. The Colosseum, originally known as the Flavian Amphitheatre, is a large amphitheatre in the city of Rome. The construction of the Colosseum started around 70—72 AD and was finished in 80 AD. Emperor Vespasian started the works, and Emperor Titus completed them. Emperor Domitian made some changes to the building between 81—96AD. It had seating for 50,000 people. It was 156 metres wide, 189 metres long and 57 metres tall. The Colosseum is a free standing building, quite different to the earlier Greek theatres which were built into the sides of hills. It is really two Roman theatres joined together. It is oval shaped, 189 meters (615 ft / 640 Roman feet) long, and 156 meters (510 ft / 528 Roman feet) wide. The Colosseum was huge. It could seat 50,000 people. It covers around 6 acres of land and is 620 feet long, 512 feet wide, and 158 feet tall. It took more than 1.1 million tons of concrete, stone, and bricks to complete the Colosseum. Construction on the Colosseum was started in 72 AD by the emperor Vespasian. It was finished eight years later in 80 AD. The Construction of the Colosseum. The Colosseum is a huge building, an amphitheater to be exact, standing 620 feet long, 513 feet wide and 159 feet high and capable of holding 50,000 people. So the total length of the Colosseum was originally planned, according to one convincing reconstruction, as 660 Roman feet long (300 + 360) and 540 Roman feet wide The perimeter can be roughly calculated as (L + W) x /2 or 1,885 Roman feet (or more precisely, using trigonometry). Entrance arches in grand amphitheatres were 20 Roman feet wide, with 3 Roman feet extra for the columns in between. So the Colosseum received a perimeter of 1,835 Roman feet (80x 23 =1840), and the arena was adjusted to 280 x 168 (still 5:3). Similar numerical patterns can be seen in the Colosseum's famous facade. façade The Colosseum was built of blocks of travertine stone extracted from the quarries of Albulae near Tivoli and brought to Rome by a wide road specially constructed for the purpose. The amphitheater forms an oval 527 meters (1,729) in circumference with diameters of 189 and 156 meters (615 and 510 ft). 
http://www.rome.info/colosseum/ http://colosseum.pbworks.com/w/page/16155807/Architecture https://en.wikipedia.org/wiki/Colloseum https://en.wikipedia.org/wiki/Colloseum https://simple.wikipedia.org/wiki/Colosseum http://www.ducksters.com/history/ancient_rome/colosseum.php http://thegrandcolosseum.weebly.com/the-construction-of-the-colosseum.html http://www.bbc.co.uk/history/ancient/romans/colosseum_01.shtml http://romancolosseum.org/roman-colosseum-architecture/
how wide is the colosseum