According to this ABC News report, the 1979 Russian anthrax accident was caused by a release of between one gram and 75 kilograms of anthrax spores. Quite a range, the Defense Intelligence Agency figured about 10 kilos. I've seen reports of 64 deaths, 100 deaths, "thousands" of deaths resulting from this leak; one report said if the wind had been blowing towards Sverdlovsk, there could have been hundreds of thousands of deaths. Of the actual casualties, some were many miles from the point of origin.

I agree that "if they expected a large number of casualties it makes whoever they are look pretty stupid", but people who could pull anything like this off have to be reasonably bright, in a twisted way. Makes me wonder if they had specific reasons for choosing the mail as a means of distribution...