Replacement V-belts must be the same length and width as the original. A belt that's too long or too short may not allow enough adjustment for proper tension. A belt that's too wide or too narrow will not ride at the right depth in the pulley grooves.
CAUTION: When installing a new belt, do not attempt to "stretch" it over pulleys. Doing so can break the internal cords causing the belt to fail. Always loosen the pulleys so there is adequate clearance to slip the belt over the pulleys.
Once the belt has been installed on the pulleys, a belt gauge should be used to adjust belt tension to factory specifications. The old rule of thumb of allowing 1/2 inch of "give" between the furthest pulleys is not a very accurate guide for today's engines. So follow the manufacturer's recommendations for belt tension.
Once tension has been adjusted, it should be rechecked and readjusted (if necessary) after a short break-in period (say after 500 to 1,000 miles of driving). It should then be checked twice a year or every 5,000 or 6,000 miles thereafter.
On vehicles with a single serpentine belt, tension is usually self-adjusted automatically via a spring loaded tensioner. No additional adjustment is necessary.
If your engine has been eating or twisting belts, misaligned pulleys may be your problem. Alignment can be checked with a straightedge. If a pulley is bent or not in the same plane as the rest, the problem should be corrected otherwise the "bad" pulley will continue to ruin belts.