Just some python 3.5 options to complete the answers above. (I would have loved to find them here).
import os
import shutil
from send2trash import send2trash # (shutil delete permanently)
Delete folder if empty
root = r"C:\Users\Me\Desktop\test"   
for dir, subdirs, files in os.walk(root):   
    if subdirs == [] and files == []:
           send2trash(dir)
           print(dir, ": folder removed")
Delete also folder if it contains this file
    elif subdirs == [] and len(files) == 1: # if contains no sub folder and only 1 file 
        if files[0]== "desktop.ini" or:  
            send2trash(dir)
            print(dir, ": folder removed")
        else:
            print(dir)
delete folder if it contains only .srt or .txt file(s)
    elif subdirs == []: #if dir doesn’t contains subdirectory
        ext = (".srt", ".txt")
        contains_other_ext=0
        for file in files:
            if not file.endswith(ext):  
                contains_other_ext=True
        if contains_other_ext== 0:
                send2trash(dir)
                print(dir, ": dir deleted")
Delete folder if its size is less than 400kb :
def get_tree_size(path):
    """Return total size of files in given path and subdirs."""
    total = 0
    for entry in os.scandir(path):
        if entry.is_dir(follow_symlinks=False):
            total += get_tree_size(entry.path)
        else:
            total += entry.stat(follow_symlinks=False).st_size
    return total
for dir, subdirs, files in os.walk(root):   
    If get_tree_size(dir) < 400000:  # ≈ 400kb
        send2trash(dir)
    print(dir, "dir deleted")