A long SMS is an SMS that exceeds the 160 characters of a standard SMS. Past this standard SMS, characters are counted differently, and one SMS becomes equivalent to 146 characters. Therefore:
One SMS = 160 characters;
Two SMS = 306 characters;
Three SMS = 452 characters, and so on…
The same rules apply to a standard and a long SMS. If you want to personalize the sender of a marketing SMS, a STOP mention of 14 characters will be added to your message. If you want to add a short-tracked link, count between 16 to 20 characters.