I did some printouts in JEmpeg during the call to ProtocolClient.restartPlayer (there is a similarly named class+method in emptool). Here's what gets sent/received:

Sent: CommandRequestPacket
Recv: CommandResponsePacket
Sent: PingRequestPacket
Recv: ProgressResponsePacket
Recv: ProgressResponsePacket
Recv: PingResponsePacket

So the size of the packet comes from the definition in packets.h of tagCommandRequestPacket, which is:

typedef struct tagCommandRequestPacket
{
EmpegPacketHeader header;
int command; // COM_RESTART
int parameter0; // RESTART_PLAYER
int parameter1; // 0
char parameter2[256]; // NULL
} CommandRequestPacket;

So there's your packet -- it should be a little more than 300 bytes I think.

Good luck -- let me know if you need more info.

Mike