Once you’ve copied one of the Debian disk images to an SD card, you may have noticed that the disk image is only for 2GB and you have a lot of unused space on your SD card. The raspi-config util has an option (Expand Root FS) to resize your root partition for you. There’s also plenty of other utils to do the job. I used gparted from a gparted LiveCD booted on my Mac to resize my partition and it worked well.