is that possible to import all possible size of font size in single spritefont file for each type of fontstyle? instead of having multiple spritefont file just for different size.
No, this is not possible. It would be cool to have a pipeline extension that handles this. One way it could work is at runtime you load a SpriteFontSet and can get all loaded fonts from it. The processor and reader can just leverage the existing SpriteFont stuff.