Ava Find does show folder sizes and it'll sort on size too. The only snag is there's no easy way to specify a location to search, any filter you set will apply to all future searches too! But, it's instantaneous and depending upon the situation could be the fastest solution.
Finding empty folders is quite straightforward in X²
- it'll let you search for folders with a filter for content=0.
I can specify byte sizes in the content field but that doesn't seem to work like it should and I don't get any results.
It's fairly simple if you break the task up though. Search for all folders under the target location --> enable showing sizes for folders found --> search these results for specific size (min,max,range). I have a feeling if you break the task up similarly it should be possible with most good file managers.
Props to Don for his attitude!