Road handlebars come in several widths and bends while Mountain bike bars can be cut to size. Traditionally the handlebar-width should correspond with the width of the shoulders which is measured by the distance between the shoulder joints. Road bars are measured from the outer extremities but note that ITM and Cinelli measure C-C (center-center). The difference is a whole 2 cm increment, so if in doubt: measure!
A wider bar has many benefits to the rider like opening the chest for better breathing, more leverage for power during sprinting out of the seat and handling a Mountain bike on the rough stuff. However a wider grip creates more frontal surface area and therefore is less aerodynamic.
A more narrow bar comes at the cost of less confidence and control over your bike, however easier for tight bunch riding for road use and easier to get through traffic for urban riders. A narrow bar for MTB is a bit a little easier to negotiate through tight woodsy type single track. You’ll need to find your own balance between the two. If you choose to match the bars in the traditional way, you will need to know your shoulder width. Measure your shoulder width here.
BRAKE LEVER AND BAR ANGLE SET UP
The neutral placement for a road brake levers is achieved with the brake lever tips in line with the bottom of the handlebar drops. Shimano STI levers seem best from the middle position of the bars, while Campagnolo’s Ergopower levers tend to get mounted slightly higher near the top curves of the bar. The alignment should be adjusted in such a manner that lower arm and hand are positioned in the straightest line possible. 
As long as you can safely reach and operate the brakes from both normal hand positions, try to bring the lever up to create a more comfortable smooth bar to hood transition.
For a Mountain bike, the levers should be directly opposite your shoulder (around 45o) for quick and comfortable reach to the levers without losing grasp of the grip or your fingers stumbling to get over the top of the lever.
Some research has been conducted into the relation between the adjustment of the handlebars and a comfortable position on the bicycle (Bremmer 1994). The most important conclusion of this survey was that the distance between saddle and handlebars is very much a matter of personal preference.
