Thanks for this plugin! Works great.
I found a few quirks, but I don't think it can be helped by the plugin, but it might help others if they are having problems getting it to work.
First, you have to enter a category or post for the settings to take hold, if you leave those fields blank the settings won't save.
Second, it does not affect images that have already been uploaded. It only works for images after the setting has been applied. I'm assuming this is because wordpress creates actual files of the different image sizes at time of upload.
Thanks again! Works well with Cleaner Gallery and the gallery short code.