The answer to this question depends upon the size of the sofa as well as the additional seating furniture that is paired with the sofa.  You want the coffee table to be proportional to sofa yet allow for enough walking space within the seating area.

A good rule of thumb is to have the table at least 2 feet shorter (30 inches shorter is optimal) than the length of your sofa.  This will allow a foot on either side of the sofa to serve as an adequate walkway.  Scale is important but don’t obsess about the actual measurements!

Additionally, when purchasing a coffee table keep in mind the style of your sofa in order to create a balanced look.  If you sofa is “heavy” looking and hovers just above the floor, you may want to opt for a “lighter” leggy looking table to open up the space.  Likewise, if your sofa has turned legs and a lot of detailing, a blocky wood table that is solid to the floor might work best.  You want the table to compliment the sofa and not compete with it.