There is a limit on how big images can be in Flash. So we limit how big the canvas can be displayed to fit within these restrictions.

When you are working with an graphical object such as an image, and the object is much bigger than the canvas, it will affect the limit of how big the canvas can be displayed, and the canvas display size may be automaticaly sized down.

If you suddenly are in a situation where the canvas gets very small and you cannot display it bigger, it's because within the canvas is a large graphical object, much larger than the canvas. To fix this, copy the part of the image that is actually visible into a new object and remove the old one.

One way of copying what is visible in the canvas to a new object is to use the menu “Pages->Current Page->Copy Page into new Image”.

Another way of doing it is using the selection tool.