Constrained proportion of a page

I’d like to set some of my pages the way that it would be just the size of the screen, both height and width, without any scrolling option.
Right now, I guess because of responsiveness, some content gets pushed down on smaller screens. As a result copyright bar jumps on loading, sometimes disappears. I’d like to have it fixed to the bottom without jumping around
At the same time, some of the page content is short but since it’s aligned to the top, the code adds up a gap underneath.