brake line size for chev cavalier - 2000

I need to replace a section of the brake line attached to the undercarriage of my 2000 Cavalier. What size line should I use? I don't have a flaring tool, but I do have a tube cutter...are compression fittings acceptable. Lastly, it appears that there may be a 2 or 3" coupling at about the midpoint of the line running under the floor plan. Hard to tell with the rust if it's a coupling of some sort or what purpose it serves. It's wider that the line running into it. What is it called and do I need it when I install the new line? Thanks