Zeiss Contax IIa 50mm Sonlar f:2.0 filter size?

How big? And is there a particular Zeiss lens hood -- also, what size?

The standard filter size for most Zeiss lenses (and Russian copies) for the Contax is 40.5 mm. There are also 42 mm push on filters and shades. The main exception to this is the 85 mm f/2 which takes 49 mm filters/shades. I recommend vented shades that can be found on eBay for under $10 over the official Zeiss shades that block a lot of the viewfinder for the normal lens.

AJG has it, I think.

 

However, the Sonnar 50mm was made for lots of different cameras, so you need to give more information (Sonnar 50mm f/2 for Contax IIa, etc.)

there are a variety of mounts, so specification of camera it's on would also be helpful

Contax IIa. (I once knew it, but the little grey cells seem to be forgetting more and more tiny little insignificant details from time to time.)