HMS Queen Elizabeth.

[NormanB]

Digging a bit further, I was clearly wrong about the Nuclear bit, but I reckon it was USS Wasp which visited Portsmouth in March 1962. The timeline is about right. Her LOA is said to be 250 meters... and I remember her alongside 'cos I could have reached out and touched her (slight exaggeration)

Then again I could have the year wrong, I'm sure it was earlier than this photo
http://c7.alamy.com/comp/HG2B56/uss-wasp-cvs-18-at-portsmouth-in-september-1968-HG2B56.jpg

Wasp was around the size of the Ark Royal (Audacious class) so very much smaller than one of the Nukes and significantly smaller than QE. Full load displacements Wasp- 41,000 , Ark Royal-45,000, QE - 70,000, Nimitz 100,00 tons (Their (The US new class of carriers are about the same displacement).