No Special Characters
Short messages with no special characters are 160 characters for the first segment. If the message exceeds 160 characters, segment length drops to 153 characters.
With Emoji, Special Characters
Short messages with emoji or special characters are up to 70 slots. A standard character takes 1 slot while the special character takes 2. So, 68 characters + 1 emoji is fine for a 1 segment message.
Long messages with emoji are comprised of 67 slots per segment.