DynamoDB: Incorrect item size calculation for Number attributes

(Posted by Claude Code)

Description

Moto's DynamoDB DynamoType.size() method computes the size of a Number attribute as len(str(value)) — the length of the decimal string representation. Real DynamoDB uses a more compact encoding where the size is approximately ceil(significant_digits / 2) + 1 bytes, as documented by AWS.

For example, a Unix timestamp like 1786461547 (10 digits):

• Moto computes: len("1786461547") = 10 bytes
• Real DynamoDB uses: ceil(10 / 2) + 1 = 6 bytes

This means moto's 400 KB item size check is more restrictive than real DynamoDB's. Items that DynamoDB accepts can be rejected by moto with ValidationException: Item size has exceeded the maximum allowed size.

Steps to reproduce

import boto3
from moto import mock_aws

@mock_aws
def test():
    client = boto3.client("dynamodb", region_name="us-east-1")
    client.create_table(
        TableName="test",
        KeySchema=[{"AttributeName": "pk", "KeyType": "HASH"}],
        AttributeDefinitions=[{"AttributeName": "pk", "AttributeType": "S"}],
        BillingMode="PAY_PER_REQUEST",
    )

    # This is accepted by real DynamoDB but rejected by moto
    client.put_item(
        TableName="test",
        Item={
            "pk": {"S": "x" * 40},
            "data": {"B": b"x" * 409530},
            "ttl": {"N": "1786461547"},
        },
    )

test()

Root cause

In moto/dynamodb/models/dynamo_type.py, DynamoType.size():

def size(self) -> int:
    if self.is_number():
        value_size = len(str(self.value))  # <-- should be ceil(digits / 2) + 1

Expected behavior

Number attribute size should be computed as ceil(significant_digits / 2) + 1 to match DynamoDB's actual encoding, per the AWS documentation on item size calculations.