Migration to Amazon Web Services... How to choose instance size? [closed]

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http://serverfault.com – We are currently using Rackspace Cloud Sites for hosting a lot of company-related websites. Because of shortcomings with the service (stuff like no SSH access...), we are considering moving to another hosting service. We are going to be evaluating Amazon Web Services and other options. For AWS, there seem to be a TON of different options for instances sizes and services. How do you know what to choose? Where do you begin? How do we determine the needs for our sites? It's difficult to judge what we are using right now since Rackspace Cloud Sites is a general blanket cloud computing service. W (HowTos)