I'm drawing an image in editing an slide show with the drawing tools. But the aspect ratio of the created drawing seems fixed and not changeable with mouse dragging the border and corner as in other applications. How can I change this ratio?
You should be able to Shift+drag the corner handles of the orange frame to change the aspect ratio. (You may need to double click to get the thick gray editing frame first.)
Note: if resizing individual elements Shift operates in reverse: holding it constrains the aspect ratio, while not holding it allows free resizing.
You can resize the canvas using Ctrl+drag and continue drawing:
The above operations are covered in Resizing, Cropping, and Adding Margins to Graphics
See the main Editing Mathematica Graphics Tutorial for many other helpful examples.