So I have followed Eric's wonderful article here: http://alestic.com/2009/12/ec2-ebs-boot-resize
This was the code basically that helped me increase the default size of the AMI:
ec2-run-sintances ami-ID -n 1 --key keypair.pem --block-device-mapping "/dev/sda1=:250"
Running Ubuntu 11.10 I didn't even have to re-size the disk afterwards, it was immediately a 250GB drive.
How do I go about decreasing the default size of the AMI???
ec2-run-sintances ami-ID -n 1 --key keypair.pem --block-device-mapping "/dev/sda1=:100"
Obviously... but I was told:
Client.InvalidBlockDeviceMapping: Volume of size 100GB is smaller than snapshot ####### <250>