I start to type in a standard IM conversation, whith a Google Talk account.
When I get to 68 chars typed, the small button to send the line gets a red x on it, and I can't press the button. I have to delete chars down to 67, and then I can press the button to send that line of text.
Of course I can send several lines, but the total of chars cannot exceed 67.
standard SMS can transmit up-to 140 bytes (per message).
when you type Unicode (multi-language) every character is represented by 2 bytes - thus 70 characters.
now, when you type only English and numbers you don't need the entire 8 bits of a byte. you need only 7 bits. so you gain 1 extra bit per byte (of the 140),
140 / 7 = 20. that "compression" adds another 20 characters to the SMS so the 140 bytes become 160 characters.
I hope it is no too technical... :-)
oh, and when you type 200 characters and the SMS is being sent without any problems, keep in mind you're being charged more than once (1 per 160 chars)