Im having trouble mastering the fiber brush. Is there a way to adjust the length of each fiber?? I just need to know some tips so i can get down to making environments (with grass) and hairy creatures.
Go to the top of the page to ‘QuickLinks’, then to the ‘Fiber Brush’ section…
I am still working on this too…but the density, gravity, and those settings under the tool modifiers section change all that as well as changing the draw pointer size…also on one of the tutorials about eyelashes and stuff like that, some of those ppl spoke of using a fine snake hook brush for the individual hairs…I normally take my models that have long hair and use the regular fiber brush and then go back and add the snake hook brush to pick out areas that i think need that look for the long hairs…I dont know how clear I am being…but hope that helps…this is a simple image that shows a little of it that I worked on while goofing off…hope it helps ya… 
I used the fiber brush on her head and then drug out the individual hairs…
Down load the ZAcademy Compile PDF.
there is other methods in there about the fiber brush like(bake and best Render)
For length, go to:
Draw 
Depth and change it to your liking. 
had the same porblem and asked for help. go to This Link.
It should help. I still haven’t mastered the fiber brush, I learned a great deal.