By default, task durations are calculated automatically, based on each task's work estimate, the calendar and the Work Estimates Configuration.

Manually Adjusting Duration

To adjust a task's duration:

Dragging the right side of a task bar will cause the task's work to be updated and its duration to be recalculated.

If you have both Start and End dates defined, work estimates will be ignored and resizing the task bar will only affect the Finish Date.