At the end of the day, a sub index is a Lucene index, and there are several options for you to get the number of documents/resources in it. You can use LuceneHelper and get the index reader out of it, and from the there, its doc count.
I am not sure that I understand what you mean by size? Do you mean the size of the index is bytes?
Regarding purge, do you mean delete the index? You can get the SearchEngineIndexManager from Compass instance and then call deleteIndex on it.