Hello Patrick !
Convert your image to 8-bit (Image->type->8bit) .
Then adjust the threshold(Image->Adjust->Threshold).
Set the scale of your image(Analyze->setScale).
Then activate the measurements you want to get (Analyze->Set Measurements--Bounding Rectangle in your case works fine to get the length of the particles --) .
Then analyze the particles (Analyze->Analyze Particles-- Adjust the parameters for your needs ! ***activate "Show Outlines"***activate "Display results" to get the values----).
Adjust everything and all to get the best results !!!
Create a macro for the whole procedure and then
download this plugin for multiple image processing:
http://ciar.rcm.upr.edu/projects/imageJ->Load and process your images with the applied macro->hopefully finished your project !
I hope this short todo helps you !
Good luck !