The root partition of an EC2 instance is taken directly from the AMI you're launching with, and it is the same size as the AMI itself. If you have a 30GB AMI, you get a 30GB root partition.
The "instance storage" described on that page comes as one or more separate scratch partitions. I don't have a running instance in front of me, but the partitions will typically show up as /dev/sdb, /dev/sdc, etc. I believe that the first is formatted as ext3 and mounted at /mnt; you have to mount (and possibly format) the other partitions yourself.
Edit: Here's what I get if I start up a Large instance using the ami-fd4aa494 AMI from http://uec-images.ubuntu.com/releases/lucid/release/.
ubuntu@ip-10-212-181-187:~$ mount | tail -n 1
/dev/sdb on /mnt type ext3 (rw)
ubuntu@ip-10-212-181-187:~$ df -h | tail -n 1
/dev/sdb 414G 199M 393G 1% /mnt
ubuntu@ip-10-212-181-187:~$ sudo blockdev --getsize64 /dev/sdb
ubuntu@ip-10-212-181-187:~$ sudo blockdev --getsize64 /dev/sdc