that's not even a clear picture of the ring. It could be a quarter carat or smaller, it's impossible to tell from this picture. As JG said, we'd need to know the finger size too....do you have another picture? Do you have any info on it?
Really hard to guess without any other info. Assuming it's like the average woman's finger size of 7, I'd guess a half carat in a halo/shank weight of another half carat, so 1 ctw?