Years ago, when I lived in Greece, I saw crows from a close distance, but never with this peculiar colouration. Today I saw two individuals, both with these gray parts.
Location: Austria, Vienna. Pretty much in the city centre. Specifically Altes AKH Hof 2 near the rear yard of the department for East Asian studies.
Time: I am not sure. Probably before my lecture (around 10am) or after it (12am). I'll see, whether I find the timestamp of the picture anywhere.
Size: Now, at this I am really bad. I would say roughly the length of my boot, which is a 47-48 European continental size.
Behaviour: nothing interesting. Two of them came, stalked around the benches, did not make sounds and as far as I remember were gone quickly. They were not afraid of people.
My guess is Corvus cornix , but I have always been bad at identifying birds.
EDIT: Unfortunately, I didn't see the linked answer. But had I seen it, I would probably have assumed my bird wasn't Corvus cornix. The plumage in the linked answer looks significantly more colourful. Whether you want to mark it as duplicate or not, I am fine with your decision, since I have little practice in identification. However, I find the answer here more detailed when it comes to differentiating C. cornix from other corvids. The linked answer only states the results. The answer here teaches us what to look for!