You should really warn people that since your plugin deletes old sizes that will break old posts that have those sizes hard coded in. That is why Viper doesn't have his do that deleting.
For example, when you add a large image size to your post the link for the image is "siteURL/blog/wp-content/uploads/2012/08/XLstates-Post-its-520x390.jpg"
With your plug in, that old 520x390 size will be deleted and therefor link the embed link for that image will be broken.
Thanks for your work, but since I didn't realize this it is causing me hours of fixing old posts.