To find the size of the cache directory uses the code below.
public void clearCache() {
    //clear memory cache
    long size = 0;
    cache.clear();
    //clear SD cache
    File[] files = cacheDir.listFiles();
    for (File f:files) {
        size = size+f.length();
        f.delete();
    }
}
This will return the number of bytes.