s3size.py

import boto3

debug = False
bucket = 'your-bucket-name'
prefix = 'some/s3/prefix/'

s3 = boto3.client('s3')

size = 0

result = s3.list_objects_v2(Bucket=bucket, Prefix=prefix)

try:
    size += sum([x['Size'] for x in result['Contents']])
except Exception as e:
    if debug:
        print(result)
    raise e

while result['IsTruncated']:
    if debug:
        print('Continuing from: ' + result['Contents'][0]['Key'])
    result = s3.list_objects_v2(
        Bucket=bucket, Prefix=prefix,
        ContinuationToken=result['NextContinuationToken'])
    size += sum([x['Size'] for x in result['Contents']])

print('Total size in Gb: ' + str(size / (1000**3)))