A detailed image on how to measure a windsock diameter with pictures is available here
Below is written instructions:
First lay your windsock flat on the ground. Measure the inlet mouth size while it is laying flat. Take this number and multiply it by 0.67 and this will give you your inlet diameter.
EX: 28" inlet mouth size lying flat x 0.637= 17.84" ≈ 18" diameter windsock
The length of the windsock can be determined simply by measuring it end to end