Choosing the right rug for your space always starts with measuring the room it will live in. While this seems like an obvious first step, you'd be surprised how many people glance at a space and take guesses.
The problem with taking guesses is that you may end up with a carpet who's scale doesn't match the space it's in - wether that is too large, or too small, the fit and feel just isn't right and now you're forced to live with it or invest in another carpet.
If you're going for the wall to wall look, then measuring the room accurately is highly important. If it's just an inch or two off in any direction, the end result you're going for will not be achieved.
Sometimes it may not be the room in particular that needs measuring, but the specific space within the room. Maybe you want to delineate separate living areas with floor coverings. One way of deciding what size rug is the best for that space would be to use painters tape to measure out your ideal rug dimensions.
If you're purchasing a new pre-made rug, it's good to keep in mind the standard room sizes available when laying your tape lines. If you're ready to invest in something that's custom made just for you and your space, simply lay the tape exactly where you want the rug to sit. If you're going for an antique rug, the best thing to do is to layout two different tape lines - one for the smallest size you're willing to use, and one for the largest size - and then try all the workable options you find in the space before committing. Even if you're using tape to decide on a rug size, it's still good to double check yourself before heading out to shop.
Better safe than sorry, right?