[iamjems]

PHP код:

public OnPlayerCommandReceived(playerid, cmdtext[])
{
    if(strlen(cmdtext) > YOUR_MAX_LENGTH)
    {
        SendClientMessage(playerid, COLORERROR, "The command you entered is too long.");
        return 0;
    }
    return 1;
} 


I have no idea why are you checking the command's length, but there you go...