"...Posterity admires, and will long admire, the awful remains of the amphitheatre of Titus who so well deserved the epithet of Colossal..."
In a footnote, he refers to Maffei, Verona Illustrata, p. iv. l. i. c. 2. I could not find this online, but instead, there is a Piranesi illustration, c.1760.
For detailed information on the Colosseum, one can go to Lacus Curtius, which has both Amphitheatrum Flavium (Platner's) and a more general Amphitheatrum (Smith's Dictionary), the latter with drawings and photos.
There is also a paper (pdf format) titled The Amphitheatre Construction Problem.