Determining the right size of runner rug for your hallway can be a bit of a challenge. You'll need to think about whether you'd like the rug to blend in or stand out, and you'll need to take careful measurements to ensure that the rug will fit in your hallway without causing a tripping hazard. While it may seem easier to just go to the store and buy a rug you like, by slowing down and doing some planning beforehand, you can ensure that your rug will work in your hallway before you buy.
To begin, determine what you want the runner rug to do. Is the rug going to hide flooring you don’t like or that isn't finished? If so, you'll want your runner to extend in width from wall to wall, covering all of the flooring you dislike. If you're installing a runner mainly to add visual interest, consider your existing flooring as a frame for your new rug. The existing flooring should peek 1 to 2 feet around the sides of your runner rug.
The length of your rug will also be determined by the function of the rug. If the runner is used by your front or back door and it's used to protect your floors from dirt and foot traffic, the rug should come as close to the door as possible. If your rug is used in an upstairs hallway, away from doors leading to and from the house, you have much more leeway. You can expose 1 to 2 feet of the existing floors at the ends of the runner before the room terminates in a doorway, or you can leave much more space and use the runner only in the center of the hallway. Measure carefully before you buy; the length will often determine the price of the rug, and you don't want to pay for rug length you don't need.