if you want to use the sms-application of the device you have to work within its limits.
you can send a sms on your own though.
code to send the message 'msg' to the number 'number' :
MessageConnection _dc = (MessageConnection)Connector.open("sms://"+number);
Message sms = _dc.newMessage(MessageConnection.TEXT_MESSAGE);
TextMessage sms2 = (TextMessage)sms;