I have the blue bag (large). The specified dimensions are listed as empty and
flat length and width, not diameter and length. I took a look at mine and indeed
it is the measured flat width of the opening, 24".
So, the cylindrical diameter is right about 15.25".
The diameter is roughly 2/3 times the flat width or exactly 2/pi times the flat
width (pi = 3.141592...). The rough diameter calcation is about 5% larger than
the actual diameter so one has to be careful with rough estimates.
A closer approximation can be reached by using 2/3 times the flat width (rough
diameter) and subtracting 5% of the rough diameter from the rough diameter;
2/3*L-.05*2/3*L or 2/3*L - 2/3*L/20. This way one doesn't have to remember pi,
only 5% or 1/20.
For a 24" diameter ball you'll need a 38" flat width bag or greater. You might
try the NRS PFD bag;
http://www.nrsweb.com/shop/product.asp?pfid=3094&pdeptid=1008 , but it's a bit
pricey in comparison though.
Answered on 10/30/2012
Bronco Billy from San Jose, CA