I just created a Large (m1.large) 64 bit instance which comes with 850 GB instance storage. Look at the Large Instance http://aws.amazon.com/ec2/instance-types/

A 'df -h' from the root folder gives me the output below. The /mnt is where I'm thinking the instance storage is but here it is only showing me 414G. I have set up two servers and both are showing the same numbers.

 root@ip-11-11-11-11:/# df -h
 Filesystem            Size  Used Avail Use% Mounted on
 /dev/sda1             7.9G  1.1G  6.5G  14% /
 none                  3.7G  112K  3.7G   1% /dev
 none                  3.7G     0  3.7G   0% /dev/shm
 none                  3.7G   48K  3.7G   1% /var/run
 none                  3.7G     0  3.7G   0% /var/lock
 /dev/sdb              414G  199M  393G   1% /mnt