Did the Coliseum get its name because of its "colossal size" or is there another explanation?

The real/original name of the Colosseum, is the Flavian Amphitheater. It took this name because it was built by the Flavian family of emperors in Rome. The theater was built next to an enormous statue of the Roman emperor, Nero. This statue was often refered to as the "colossus of Nero". The name of the statue was eventually misinterpreted or misread, as "Colosseum". The statue of Nero (colossus) was eventually torn down, but the name colossus or Colosseum remained, and was used to refer to the Flavian Amphitheater. In essence, the Colosseum got its name from a nearby statue.

https://en.wikipedia.org/wiki/Colosseum

Colossus was a Latin word, meaning any statue that is larger than life size. They spelled it "Colossos" but then changed it to "Colossus."

Medieval Latin Colisçum, an amphitheater in Rome, Italy, variant of Latin Colossçum, from neuter of colossçus, gigantic, from colossus, huge statue; see colossus.

Colosseum -- Medieval L. name for the classical Amphitheatrum Flavium (begun c.70 C.E.), lit. neut. of adj. colosseus "gigantic;" perhaps a ref. to the colossal statue of Nero that long stood nearby (see colossus).

http://www.etymonline.com/index.php?search=colosseum&searchmode=none

http://dictionary.reference.com/browse/coliseum