Ciao Whirlybirdguy!
How much do booms extend out from helicopters.
The boom width varies from one aircraft type to the next and from one spray-gear manufacturer to another.
On the Bell 206 using the standard Simplex booms, the boom span is 36ft with a rotor diameter of 33ft.
What is the width covered in each run by most helicopters.
The 'width' of the chemical (or fluid) being applied is referred to as the swath and again .. this varies according to boom width, height and speed flown, the density of the fluid/chemical, wind conditions, pilot technique, nozzle aperture and fluid dispersal rate.
What I suggest is that you try and obtain as much information as you can about your requirement, the area to be sprayed, what it is that you are spraying, the type of terrain involved, the elevation and temperature etc. and then contact one of the many spray companies (you can find a partial list of some of these companies on the previous page in post #95) and seek their advice.