Hoping someone can help me with the socket size for my spark plugs on a 1200, 1964 beetle.
The thread size is 16mm but not sure what the socket size is.

I think you'll find the thread size is in fact 14 mm. The socket size is 20.8 mm (oddly enough), or 13/16".

Odd that a German company making German plugs for a German car would use a non-metric spanner size.

There's a lot of information available at e.g. http://www.bosch.com.au. 

hth

Hey thanks for that modulus. I'll be heading out to this morning to get myself a 13/16 sized socket.