hey brecht. Sorry, ive been away or else i'd have replied sooner.
Grass is fun. To create a difference in the length can be done in a number of ways.
You can either paint the length, or add randomness. Or add another fur description, but creating another fur description only clogs things up. You can do this without doing that.
Ok, personally the best way to do this is by painting the length.
Select your fur (grass). Go Fur > Paint fur attributes tool and click the option box. Select length, and simply paint on areas where you want grass taller.
Alternatively, for a bit of randomness, select the grass, go to your attr. editor. Go to grass > then down to Details > The length.
This opens up some options. Simply play with the noise attributes to get randomness.
Go to http://www.pixalbeat.com/coyotefur.html.
I did a tutorial on grass for noobs a while back to create realistic grass.
Hope this helps
those who succeed are only the failures that never gave up.