Here is my list of the largest single syllable cities (in English) in the world by rounded off metro population (when known).

Seoul, South Korea – 24,200,000

Rome, Italy – 3,700,000

Prague, Czech Republic – 2,300,000

Leeds, UK – 2,300,000 (thank you, Warren)

Perth, Australia – 2,000,000

Minsk, Belarus – 1,850,000

Lille, France – 1,240,000

Omsk, Russia – 1,140,000

Fez, Morocco – 1,110,000

Perms, Russia – 1,060,000

Qom, Iran -1,050,000

Hims, Syria – 1,050,000

Nice, France – 1,000,000

Bern, Switzerland – 660,000

Cork, Ireland – 518,000

Van, Turkey – 500,000

Flint, Michigan, USA – 426,000

Sparks, Nevada, USA – 425,000 (with Reno)

Kursk, Russia – 415,000

Split, Croatia – 350,000

Bonn, Germany – 325,000

Brest, France – 300,000

Reims, France – 292,000

Mau, India – 279,000

Durg, India – 268,000

Brugge, Belgium – 255,000

Mage’, Brazil – 244,000

Kiel, Germany – 240,000

York, United Kingdom – 202,000

Mainz, Germany -200,000

Cairns, Australia – 153,000

Neuss, Germany – 151,000

York, Ontario, Canada – 150,000 (part of larger Toronto metro area)

Slough, United Kingdom – 142,000

Nimes, France -140,000

Ulm, Germany – 123,000

Furth, Germany – 116,000

Lund, Sweden – 110,000

Moers, Germany – 105,000

Sources: en.wikipedia.org and http://www.sporcle.com