That is totally untrue.
First of all, anthrax spores ONLY COME IN ONE SIZE. They are roughly 1 micron in diameter, and 1.5 microns in length. Milling down a spore to get it to the right size would be like milling down an ostrich egg to create a chicken egg. It's idiotic. It's impossible.
Second, the anthrax spores in the letters WERE NOT MILLED. Milling is done to break up clumps of spores which were dried together. Milling leaves traces. When you mill spores, you get chips of spores in the powder, and most of the spores would appear to have flat sides as a result of the milling. (If you have two or more spores stuck together, and you separate them by milling, the area where they were stuck together will be flat.) Everyone who looked at the spores said there was no sign of milling.
The nonsense about milling was just a misunderstanding by people who knew nothing about such things. It was probably the DUMBEST thing ever said about the anthrax, and it was said by MANY people who just didn't know what they were talking about.
Ed
There was an article about this posted just a week or so ago which claims otherwise and specifies the range within which these spores must be in order to be effective when inhaled. And it claims these were within that range.