There is a maximum payload that you can send at one time. Use the NP (Maximum Packet Payload Bytes) command to read the device's maximum payload.
These maximums only apply in API mode. If you attempt to send an API packet with a larger payload than specified, the device responds with a Transmit Status frame (0x89) with the Status field set to 74 (Data payload too large).
In Transparent mode, the firmware splits the data as necessary to cope with maximum payloads.