using AT-commands (GSM-device or mobile attached with PC)
May be your device allowing 128
chars only in a single message.
you can split long message in two or more sms. because in this case(for AT-commands),
if your device support only 128 chars per sms then there are no extra settings available to extend the limit from 128 to 160 chars in your current device.
you can also use 3rd party bulk sms service for this...