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See the LA Times article "New Crop-Dusting Restrictions Weighed", from Sept. 25, 2001. [Click on the link to read the whole article.]

Here are four of the points I found of particular interest [paraphrased], and a question:

There are vents on crop-duster tanks. For this reason, nerve gas (or other gases) couldn't be used with the standard equipment, since the gas would come out the vents.
It requires special training to fly a crop-duster. These planes have an extra wheel under the tail (so that it can be controlled better while carrying heavy loads), which makes taking off, specifically, quite different from taking off in a normal plane.
Standard crop-dusting nozzles deliver their chemicals in large droplets, which is not what would be desired for biological or chemical weapons (since an attacker would want any such agent to stay airborne for as long as possible, and, even if inhaled, large droplets would probably get coughed out before reaching the lungs). Standard nozzle size is 200 to 350 microns.
Some spraying of pesticides targets mosquitoes or medflies. This spraying uses finer nozzles that would be more effective for use with a biological or chemical weapon. The nozzle size used is only 15 microns or smaller.

Here's the question: Has anybody seen figures on the size of the particles in the anthrax mailed to Sen. Leahy (which was said to be finer than any previously known)?

No one knows what size the anthrax particles were when mailed - only what they were when received.

This stuff was so finely separated that we have had reports that it was down to a single spore!

Ken Abilek has suggested that highspeed mail processing equipment may have ground the powder even finer. Letters start and stop repeatedly, at high speeds, in this equipment. It only makes sense that this would affect the contents.

Recognize also that cropdusting equipment presumes the active ingredients will be dissolved in some liquid medium (e.g., water) in order to properly disperse the contents.

The dilution rate for tractor drawn spraying equipment is in the order of 30:1. In aerial spraying, the dilution rate is something less (5 or 10:1). Accordingly, whatever is to be sprayed will be a.) in liquid format and b.) diluted with water (or some other solvent).

These conditions may or may not be conducive to anthrax in coated powder form. But they may be particularly suitable to some other biological or chemical agent.

The use of aircraft (e.g. crop dusters) is one way to deploy chemical and biological weapons. But as has been correctly noted in this tread it can be complicated based on the type and quality of agent, nozzle technology (often a big problem), wind and other factors which may degrade the effect. The point is that al-Qaeda was clearly investigating this. Whether it is an effective terrorist method or whether or not it was ultimately used is another matter.