R2K's High Power Rocketry features a bunch of images/diagrams of Convair's proposed Nexus. I linked to one of them. For me, this gave a better indication of its size than a list of specs. But, here they are anyway (via Astronautix):
Status: Design 1962.Note that Nexus would have been a reusable VTOL vehicle!
Gross mass: 21,820,000 kg (48,100,000 lb).
LEO Payload: 450,000 kg (990,000 lb).
Height: 122.00 m (400.00 ft).
Diameter: 45.70 m (149.90 ft).
Span: 50.00 m (164.00 ft).
I noticed Astronautix attributes the design to General Dynamics. If anyone can clarify this, please leave a comment. I thought I may have seen this vehicle covered in one of luke strawwalker's summaries on YORF, but I have no inclination to research it at the moment.