calculating S3 file size containing <your_filter> in <your_bucket> using boto3 python library.

calculatingS3Size.py

import boto3
s3 = boto3.resource('s3')
bucket = s3.Bucket('<your_bucket>')
size = 0

for o in bucket.objects.all():    
    if '<your_filter>' in o.key:
        print o, o.size
        size += o.size    
print 's3 size = %.3f GB' % (size/1024/1024/1024)

@nberserk
Thank you very much!
And we can get one file size with this code:

for onefile in bucket.objects.filter(Prefix='<filename>'):
  print(onefile.size)