If you find that space somewhat limiting, it is easy to give yourself a larger root volume at instance creation time. Its easy
Launch the instance with appropriate block-device-mapping arguments
$ ec2-run-instances $AMI --key mykey --block-device-mapping /dev/sda1=:20
That will create you an instance with a 20G root volume. However the filesystem on that volume will still only occupy 8G of the space. Essentially, you'd have 12G of unused volume at the end of the disk.
Resize the root volume
if you've launched an 11.04 based image newer than alpha-2, this step is not necessary. Cloud-init will do it for you. It is just assumed that you want your root filesystem to fill all space on its partition. I honestly cannot think of a reason why you would not want that.
Now, if you are using 10.04 or 10.10 images, you can resize your root volume easily enough after the boot. Just login, and issue:
$ sudo resize2fs /dev/sda1
That operation should take only a few seconds or less, and you'll then have all the space you need.