how do i know the default width and height of an image of an animate cc fla file inside animate cc

There is none but I would recommend HD 1920x1080. Because animate is a vector based program you can export at any size later. 

forgot to mention that it has nothing to do with importing either

there's no default image w x h. the w x h are the image's w x h when imported.

"the image of an animate cc fla file inside animate cc" makes little sense, but if you did snap an image of an animate cc fla window and then import that image into animate cc, the first sentence applies.