When adding the text width parameter to a node, this is similar to the node contents being within a minipage. As such, with text width set you can use any normal way of centering text, such as \centering or the center environment.
If you want the remaining text to be justfied, add align=justify to the node options as well, the default is to set the node text ragged right.
\documentclass{article}
\usepackage{tikz}
\usepackage{lipsum}
\begin{document}
\begin{tikzpicture}
\node [draw,text width=5cm]
{{\centering Stuff \par}
\lipsum[1]};
\node at (6,0) [draw,text width=5cm,align=justify]
{\begin{center}
Stuff
\end{center}
\lipsum[1]};
\end{tikzpicture}
\end{document}

this the body of the node spanning several lines. };
– percusse Oct 02 '14 at 09:48You need an empty line afterNode titleotherwise\centering` won't work.\documentclass{article}\usepackage{tikz,lipsum}\begin{document}\begin{tikzpicture}\node [draw,text width=5cm,align=justify] {{\centering Stuff \par}\lipsum[1]};\end{tikzpicture}\end{document}This isn't really different from normal text, anodewithtext widthset is something like aminipage. – Torbjørn T. Oct 02 '14 at 10:07