maximum field and item size limits in dynamo db and mongo db
The MongoDB limit is currently 16MB per document, this has increased a couple of times over the course of development, but that is the limit as of the current release (2.0.x at the time of writing), see here:
http://www.mongodb.org/display/DOCS/Documents
I'm not as familiar with the various limits in Dynamo, but you can find a list of them here:
http://docs.amazonwebservices.com/amazondynamodb/latest/developerguide/Limits.html
DynamoDB max per item is currently 64k (which is seen by many as a serious shortcoming). Best practice for documents larger than 64k would be to store a pointer to document (S3?) instead of the entire document, but this obviously has some associated issues as well.