"The cornerstone was laid in 1848, site problems having caused delays; then, when the monument was some 150 feet high, the Civil War postponed further constuction. Mills died in 1855. In 1876, when construction recommenced, George P. Marsh, who was ambassador to Italy, was among those asked for advice concerning completion of the monument. As a student of Egyptian obelisks—there are thirteen in Rome—he immediately suggested that Mills's projected height of 600 feet be reduced to the standard Egyptian proportions of ten times base to height, or 555 feet, a 55-foot width having been established. Marsh also strongly recommended that the circular 'temple' base be eliminated and that no decorative trim be used. Finally, he proposed a pyramidal capping of aluminum, a pioneering material for the time."

— from G.E. Kidder Smith. Looking at Architecture. p130.

Details

At 555 feet tall, one of the highest all-masonry towers in the world. 55 feet wide at the base.