This feature is only available in the desktop editor.
ScreenSteps has four handles around the sides of an image content block 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).
If your image content block has a single image then an image resize handle will appear in the bottom-right corner when the content block is selected. Clicking on this handle will allow you to resize the image. Hold down the Option (OS X) / Control (Windows) key while resizing to resize in 10% increments.
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.
If you want more precise control over the image size then you can use the image contextual menu. Right-click on the image and select Resize Image....
After selecting Resize Image... a dialog will appear that gives you precise control over the size of the image.