Area measurement of image

[janaka kumara]

Dear all,
 
I am working on particle size distribution of gravels, there I try to measure area of a particle (or particles) on an image using ImageJ tool kit.
 
First, image is made "binary" and then using "analysis particles", area is measured (keep outline in "show"). However, actual area and measured area are different (that can be easily seen from the actual image and image comes after analysing). My problem is how to identify actual boundary of the particle and measure it.
 
Can anyone please advice me how to measure actual area of the particle? 
 
Thank you,
 
Regards,
Kumara