If you go through the process of getting measured for a custom-fit bicycle, this may result in a recommendation for the "ideal" size handlebar. However, there is no "right" or "wrong" when it comes to handlebar fit; it all comes down to your personal preference for comfort and performance. As with most things when it comes to bike parts, there are trade-offs, i.e. each choice you make has some advantages and disadvantages.
In general, a wider handlebar will feel more stable and secure, because the wider the bar, the more leverage you have in controlling the motion of your front fork and wheel. A narrower handlebar will feel more responsive, because the shorter the handlebar, the less movement is required in your arms and hands to get the same amount of steering action. Keep in mind that these are relative descriptions of the steering characteristics, and different riders will have different preferences based on their riding style. For example, most racers prefer a narrower handlebar, in order to get quick, responsive steering for race-day conditions. However, a more casual rider might call that same narrow handlebar too unstable, or "twitchy."
Another advantage of a wider handler is that by placing your hands and arms further apart, this tends to flex out your chest muscles more, and "open up" your rib cage, resulting in better oxygen flow to your lungs and less fatigue during long, epic rides.
The width of handlebars on traditional drop-bar style road bikes is measured in centimeters. Most men's bikes come with a handlebar that is typically about 42cm wide, or possibly 44cm wide for larger frame sizes. Some women's-specific road bikes will come with a narrower handlebar, say 38cm or 40cm. Most manufacturers use a "center-to-center" measurement, meaning that the width is measured based on the middle point of the handlebar tubing, rather than the outside edge, but this is not the case for all companies, so be sure to verify this when making your selection.
Road drop handlebars are also available with a variety of drop shapes. If you'd prefer not to lean over as much, you can select a handlebar with a more shallow drop. Most drop bars have an ergonomic shape to fit your hands more comfortably in the dropped sections, but many fixed-gear track bikes still come with a more traditional rounded shape in the drops. Also, handlebars with drop sections that flare outward have recently started to become popular among cyclocross racers and long-distance cycle tourists. If you are selecting a bar of this type, be sure to note whether the width measurement is done at the top (non-flared) part of the handlebar, or the bottom (wider, flared) part.
If you are outfitting a road bike to be used for fully-loaded touring or commuting, use a wider handlebar even if you are accustomed to a narrow one for regular road riding. You'll appreciate the extra control and stability once the bike is loaded up with your camping gear, spare clothes, food, laptop computer, etc.
The Raleigh Sojourn touring bike has a wide handlebar with a
31.8mm clamp and flared drops (photo from Dirt Rag Magazine).
31.8mm clamp and flared drops (photo from Dirt Rag Magazine).
Let me relate a bit of personal experience to illustrate how your choice of handlebar width can affect your cycling performance. I didn't pay much attention to handlebar width until a couple of years ago, when I was looking at options to upgrade my road racing bike, which I had bought in 2001. I measured the original bar, and was surprised to find that it was only 40cm, which I thought was pretty narrow for my 56cm bike frame. A 42cm handlebar might have been a more common choice for someone my size, but I decided to go with a 44cm just for kicks. Since then, I have found that the extra stability and control lets me ride much more aggressively, and not slow down as much on the descents and corners, and I end up riding faster overall. So, this is a perfect example of how the performance of your bike can be affected by psychological factors more so than any mechanical factors.
On hybrids and flat-bar road bikes ("fitness" bikes), comfort is usually the overriding concern, so these typically come with very wide handlebars of about 56cm to 70cm.
For mountain biking, you have the same dilemma as with a road bike of balancing the quick handling of a narrow handlebar versus the stability and control of a wide handlebar. For riding in technical, tight, twisty singletrack, you have the additional factor that sometimes there are trees spaced so closely next to the trail that a wide handlebar literally will not fit through! In the earlier years of mountain biking, trends in handlebar width seemed to vary from season to season, like trends in the length of mini-skirts. However, with the recent popularity of downhill, freeriding, all-mountain, and other types of more "extreme" mountain biking, the wider handlebars seem to have established themselves as the preferred choice for most modern mountain bikes. Mountain bikers who still have a preference for narrower bars might use something in the range of 54cm or 55cm. A typical cross-country (XC) rider these days will have something like a 58cm to 60cm bar, while some downhill racers might even have a handlebar as wide as 70cm or 80cm.
Another important measurement in handlebar selection is the clamp size. This refers to the diameter of the tubing in the center of the handlebar where it connects to the bike's stem. Older BMX bikes and some kids' bikes use a 7/8-inch (22.2mm) clamp. One-inch clamp (25.4mm) was the de facto standard for mountain bikes for many years, as was 1-1/8 inches (26.0mm) for road bikes. Over the past couple of years, racers and other cyclists that demand high performance called for something to provide more strength and less flexibility in the handlebar. As a result, many of the latest bikes (both road and mountain) are using an oversized clamp, which measures 1-1/4 inches, but is more commonly known by its "31.8mm" metric equivalent.
If you are selecting both a new handlebar and stem, be sure to choose a stem with a matching clamp size to the handlebar that you want. If you are upgrading your handlebar but want to keep your existing stem, but sure to choose a handlebar that has the same clamp size as your current stem. If you are upgrading to a handlebar and stem with a larger clamp size than your current ones, be aware that you may have issues with the compatibility of any handlebar-mounted accessories that you are using, such as computers, lights, horns, and brake levers.