I am sorry that if this is a very stupid question.
I checked the documentation again and again about the Working Area. It seems that it never mention how to set the image size of the Working Area. Here's what the documentation says:
Although every project has a fixed image size (for example 640x480 pixels), the working area is basically an endless flat space on which you can draw and animate. The blue visibility rectangle mentioned above indicates the actual image area for the project.
First of all, asking something you don't know isn't stupid. Second, images are imported in their original size. The blue rectangle indicates the size of your project (what will be visible inside the rectangle of your animation). If the size of your image is larger than the size of your project, you will see the image only partially in the exported animation. There are two options here if you want to the have the whole image in view:
change the project size before you import the image, through the menu: File -> Project Settings... and then adjusting the width and height
after you have imported the image, resize the image layer with the Scale Layer tool
Scale Layer tool