Suppose I have two nodes (A and B in example) already defined. Suppose I want a third node C to be (e.g.) below node A and stretching so far to line up with node B.

```
\documentclass[tikz]{standalone}

\usetikzlibrary{positioning}
\begin{document}

\begin{tikzpicture} 
  % two example nodes, position and size do not matter: 
  \node [minimum width=2cm, minimum height=3cm, draw] (a) {A}; 
  \node [right=1cm of a, minimum width=2.5cm, minimum height=1.5cm, draw] (b) {B};

  % new node to align and size: 
  \node [below=1cm of a.south west, anchor=north west, draw,
  minimum width=5.5cm, % <--- this should not be necessary!! 
  ] (c) {C}; 
\end{tikzpicture}

\end{document}

```

I can get this done by manual calculation of minimum width, but that is of course ugly. I could define new coordinate like ([yshift=-1cm]b.south east) and then do fit, but that is ugly as well.

Is there a concise, easy to write and read, relative way of doing that?