When you add a new task to a workspace it will be allocated a one day duration by default. To change the duration you need to give the task a start and end date.

To set a task's duration click on the calendar icon that appears in the task list, or in it's detail panel, and the date picker will launch.

The first date that you click will be set as the start date, the next date you click will become the task's end date. The duration will be calculated automatically.

If you have selected 'Automatic work-day correction' in the task workspace options, the end date will be adjusted accordingly.

You can also adjust the duration of a task on the 'Timeline' view by clicking and dragging either end of a task’s bar; unless you have Automatic work-day correction selected.