A little research on all wheels mentioned. This figure is not the diameter of the wheel, but the construction's height.
Singapore Flyer is at the top with 165 metres.
London Eye next at 135 metres.
Melbourne Observation Wheel, 120 metres.
Riesenrad, Vienna, 65 metres, and a very impressive 117 years old.
Brighton, England, 50 metres.
Other tall wheels:
High Roller, Las Vegas, just opened, 168 metres.
Star of Nanchang, China, 160 metres.