When you go into the send dialo (where you can select the file and name to be sent and send to), there's a drop down box with a set of numbers in it. That's the DCC Packetsize. The highest value for it currently is 8192 and goes down by the following options:

8192
4096 (mirc's old maximum)
2048
1024
512

Those who fail history are doomed to repeat it