Don't use button objects as they are different depending on the platform. Use a sprite as your button and use the touch object to detect touches on it.
For the screen issue there are plenty of posts on the forums about it. Use request fullscreen and one of the scaling modes (scale outer, letterbox, etc...). You want to make your layout sized to fit the smallest screen size you plan to support, but enough overflow (surrounding graphics) to cover larger screens as well. ArcadEd has a great tutorial on this on Udemy.
i've viewed his video
and what i got is that we need to guide the screen focus to the middle of our layout so when we scale it outer, it still cover the screen, right?
also, what's the best resolution for android games ported using cocoonjs?
I mean the layout and the window size and fullscreen mode