Hi Mou,
Yes, a good question.
It has been a little while since I used the SMAC, but if I remember correctly, the SMAC puts the transceiver into packet mode. The maximum payload for the radio frame is 125 bytes. See the datasheet for the device, chapter 4.1.
It may be possible for you to increase the size of the transmitted and received packets from the MCU perspective, but these are needing to be fragmented before they are sent, or once they have been received. For that reason, I think that it is going to need a quite some thought for you to achieve what you want. The SMAC is going to stay the same, and you'll have to introduce your fragmentation management at the application layer.
Good lck,
Mark