Fog has the ability to define the size of your root EBS device but Chef does not support this. We have developed a patch to provide this feature.
Note: you should run resize2fs (or an otherwise appropriate tool) after build or else your instance will only use the default EBS size for your AMI. We did not automate this as it seems to require making some assumptions. This could be handled in a recipe or custom template, or simply entered in the first time you login to an instance, depending on your needs.
Will submit a pull request via github.