How to know the root device size of some public AMI?

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http://serverfault.com – Since I do not want to pay money for my testing, the free limit size is 10G. I can know the root device root for some default AMI such as Amazon Linux AMI 2012.03 The Amazon Linux AMI 2012.03 is an EBS-backed, PV-GRUB image. It includes Linux 3.2, AWS tools, and repository access to multiple versions of MySQL, PostgreSQL, Python, Ruby, and Tomcat. Root Device Size: 8 GB And the last row displayed 8GB. However, if I find AMI in Community Page, it does not show the root device size. Can anyone know how to use the instance such as centOS that is not provided by default but it is under 10GB so (HowTos)