File size is only specified for files. So, if you specify a directory as your source, you have to iterate through the directory. The below snippet should work (and should be faster than the other solutions).

import glob
 
def get_directory_size_in_bytes(source_path: dir, pattern: str = '**/*.parquet') -> int:
    source_path = '/dbfs/' + source_path.replace('dbfs','').lstrip('/').lstrip(':').rstrip('/')
 
    files = glob.glob(f'{source_path}{pattern}')
    directory_size = sum([dbutils.fs.ls(path.replace('/dbfs/',''))[0].size for path in files])
 
    return directory_size