A task or volunteer activity length can be changed in any view except Month view by moving the cursor to the top or bottom of the task or volunteer activity. The cursor will change to a double-headed arrow and you can drag the starting or ending time to whatever you want.
You can also double-click on the item in the calendar and edit the dates and times (or any other information for the task or volunteer activity) by hand. When you save the task or volunteer activity, the calendar is updated.