The size of NDEF is limited by the message format to 2^32 - 1 bytes = 4,294,967,295, so approximately 4 gigabytes. However, records can be chained together in a message to form longer payloads, so in theory, there is no limit to the length of an NDEF message.
However, it also depends on how much RAM you have available and the application you're running. If you find out how much RAM your application uses that will give you an indication of the limitations for the NDEF size.
You need to change this define in sdk_config.h if you increase the size over 1024 bytes.
