How do I make use of the full SD-card after installing ev3dev
If you install ev3dev with an image file dated 1 May, 2015 or later, it should automatically expand the root partition to fill the space available on the SD card. Older releases are no longer supported.

If it does not do this, you should report a bug a https://github.com/ev3dev/ev3dev/issues. Be sure to read http://www.ev3dev.org/support/#github-issues first.