At extreme sizes, most pages will fall apart to some extent.
When the viewer's font size is vastly larger than the average, a page will very often break up. Pages I design will work with the font size considerably larger than average.
When the viewer's browser window is much smaller than average, some page elements will probably not fit in the window. How they will arrange themselves depends on many factors. I code my pages to work from 800px and wider, but many will work with windows as narrow as 640px and sometimes even narrower.