I would suggest having a read of
http://www.globalsecurity.org/wmd/li...-14/index.html specifically chapter 2, page 17.
It doesn't give burst radius etc, but does have some interesting (and very scary) information on expected casualty rates per number of rounds in an area.
For example, at 20 degrees celsius, it only takes 4 VX bombs in an area of 150 hectares (a little over half a square mile) to inflict approximately 21% casualties. Drop 16 over the same area and you're up to 45%.
Naturally that's just a guide. There are an endless number of factors which will impact upon the final result.
What might be of more use is the statement "Agent clouds will produce very narrow and very long hazard clouds. Dosages over 100 times the lethal levels are possible in the hazard area."