I’m thinking back to the evidence the U.S. showed to the U.N. all of it was quite damning in terms of showing they had “some” weapons (phone transcripts of people saying, "Don’t use the word “Vx in our conversations”, sat photos of decontamination trucks outside of suspected weapons sites, convoys of vehicles leaving suspected sites before U.N. weapons inspectors arrive). But none of that gives us any hard data as to the quantities involved. A decontamination truck could be there because they were moving 55 gallons of the stuff.
But certainly the Bush Administration suggested large quantities, so I think from the standpoint of their credibility, finding one or two shells or a bag full of precursor material or whatever just won’t do.
Biological materials could be present in much smaller quantities and still be the ‘smoking gun’ because they would imply a pretty big threat. Probably less than an ounce of Anthrax crippled the postal system for a while, and cost hundreds of millions in cleanup, lost labor, etc. If Iraq had say, 100 lbs of the stuff, that could do quite a lot of damage if distributed properly.
Chemical weapons I would expect to see in much larger quantities. A couple of drums won’t cut it, I don’t think.