This feature is only available in the desktop editor.
Canvas resize handles
ScreenSteps has 4 handles around the sides of the step image editor that will resize the canvas. Clicking on any of these handles will allow you to change the size of the canvas where images and annotations can be placed. You cannot make the canvas smaller than an image (e.g. you cannot crop an image using the canvas handles).
Resizing an image with the image resize handle
If your step has a single image then an image resize handle will appear in the bottom-right corner when the step is selected. Clicking on this handle will allow you to resize the image. Hold down the Option (OS X) / Alt (Windows) key while resizing to resize in 10% increments.
Resizing an image with the image tool
You can also use the image tool to resize images. With the image tool selected (1), you can click on an image and handles will appear around the image (2). You can click on the handles to resize the image.
Resizing an image using the contextual menu
If you want more precise control over the image size then you can use the image contextual menu. If the step only has a single image then you can right-click on the step image and select Resize Image....
If your step has multiple images then you must select the image tool and then right-click on an image and select Resize Image....
After selecting Resize Image... a dialog will appear that gives you precise control over the size of the image.