Width is measured at half the length of your board. The widest point on your board should generally be between 18 ½ inches and 22 inches wide and be located any where from 5 inches behind center to 8 inches in front of center.
       Wider boards carry more foam, which increases flotation, allowing you to paddle faster and catch waves earlier. Wider boards are better for smaller surf allowing you to create more speed because of the increased bottom planning area and because the board is sitting more on top of the water rather than dragging through it.
      Narrower boards are suited better for bigger faster waves where there is an abundance of energy. Because they “cut” into the water the rails will give you more control in fast hollow waves. Narrower boards can also be transferred from side to side faster and easier so you will often see narrow boards used by competition surfers.

Nose Width is measured 12” from the nose of the board.
     A wider nose has more foam which increases the paddling ability and the ability to catch waves. Wider noses are better for smaller surf because the wider nose resists sinking and plowing the water as you move forward. Generally the wider the nose the less nose rocker you want the board to have so you're sure not to plow water and slow down.
     In bigger surf especially with offshore winds you would want a narrower nose so that the wind doesn't catch under the nose and keep you from being able to drop in.

Tail Width is measured 12" from the tip of the tail.
     A wider tail has more foam which increases the paddling ability and the ability to catch waves. The extra planning area of a wider tail will enhance low speed response and the stability of your board. A board with a wider tail will be suitable for bigger surf because the minimal planning area of the tail easier to turn at high speeds.