The SmartClient uploads and download the records in batches of 50 at a time for upload and 25 for download. SmartClient doesn't have a fixed ceiling on the number of records it could upload. If you try to upload 1000 records, SmartClient will do so by picking up 50 records at a time.
NetSuite, however, has a 5MB limit on the total size of the request/response it will accept. This size would depend on a combination of factors: number of records in the batch (fixed at 50), Number of fields, and the contents of those fields. Sometimes even though the number of records uploaded/download are less, the Web Services request sent to NetSuite may cross 5MB ,in such cases you will see this error "response stream exceeded limit of 5242880 bytes"
You can resolve this by decreasing the default upload/download batch size of SmartClient by creating a custom record in NetSuite by following these steps