Why are Facebook images small?
Unfortunately the image sizes returned by Facebook within a feed are inconsistent. Sometimes a full-size image is returned, but many times Facebook returns a small image in the RSS feed, which is what BlogFeeder uses to import your posts from Facebook. There's no way for BlogFeeder to fetch anything other than what's included in the RSS feed, so Facebook would have to return a larger image in order to include a larger image in the imported post.
We've been unable to get an answer from Facebook why this is the case, but we're watching for updates in case the RSS feed allows for fetching full size images in the future.