Measure (or sometimes "The Measure") in typography is the length of a line of text. For a single-column design measure should ideally lie between 40 & 80 characters. Many typographers consider the perfect measure to be 65 characters. If the lines are too short then the text becomes disjointed, if they are too long the content loses rhythm as the reader searches for the start of each line. Punctuation should preferably hang outside the measure. 
Measure and Legibility
The measure of a text influences legibility. It is relative to each design, but can be used as a unit of measurement within that design to create unity and harmony. Long lines are hard to read, short lines are more easily read. 45–75 characters per line are regarded as the ideal range for the measure. For multiple column setups, 40–50 characters are often preferred. 66 is sometimes considered ideal for one column setups. Generally, if the measure is wide, the leading of a text should be increased—if the measure is short, it can safely be decreased. Reverse text, i.e. white text on black also requires more leading.
The measure or the length of a legible line is the length of the alphabet (Caps, Lowercase and Numerals) of a typeface of a size being used.