Could it be 70 characters per line? 70 words would make a very long line.
If so, the physical line width depends on what font you're using. A monospaced font, such as Courier/Courier New/Dark Courier is 10 characters per inch, so 70 characters is 7 inches. On an 8 1/2 inch page, 1 1/2 inches will be left over, which gives a 3/4 inch margin for each side.
With a proportionate font--anything from Arial to TNR--there's just no way to say. Different letters take up different amounts of space. "W" is much wider then "i". On top of that, it varies from one font to the next. An Arial "W" isn't the same size as a Verdana "W", and neither is the same size as a TNR "W".
ETA: Courier is 10 characters per inch at the 12 point size. Different point sizes are different widths.