Sections can be of any length. They are initially given a length of four bars, but if you record past the end of a section, it will grow to accommodate the notes you play in. If the section has been placed in a song and it butts up against another section, FreeStyle won’t automatically grow the section any more.
To adjust the length of a section, drag the heavy, black double-lined vertical end bar displayed in the Graphic Editing window in the note grid. You can also adjust its length anywhere it appears in the Arrangement window grid: just grab the right edge of the section and drag it.