Can we define maximum width for a node?

I guess there is no TikZ way, but you can use the varwidth package:

\documentclass{article}

\usepackage{tikz}
\usepackage{varwidth}

\begin{document}
\begin{tikzpicture}[node distance = 2cm, auto,->=stealth,point/.style= 
                   {circle,fill=red,minimum size=0pt,inner sep=0pt}]
\tikzstyle{block} = [rectangle, draw,thick,fill=blue!0,
    text centered, rounded corners, minimum height=1em]
\node [block] (start) {\begin{varwidth}{15em}Start\end{varwidth}};
\node [block,below of=start] (start) {%
   \begin{varwidth}{15em}
      Start and blah blah blah blah blah blah
      blah blah blah blah blah blah blah blah blah
      blah blah blah blah blah blah blah blah blah
      blah blah blah blah blah blah blah blah blah
    \end{varwidth}};
\end{tikzpicture}
\end{document}

You may wrap the {varwidth} environment in a shorter macro …

Update

It is possible to use execute at begin/end node to include {varwidth} in the sytle definition:

\begin{tikzpicture}[node distance = 2cm, auto,->=stealth]
\tikzstyle{block} = [%
   rectangle, draw,thick,fill=blue!0,
   text centered, rounded corners, minimum height=1em,
   execute at begin node={\begin{varwidth}{15em}},
   execute at end node={\end{varwidth}}]
\node [block] (start) {%
      Start
};
\node [block,below of=start] (start) {%
      Start and blah blah blah blah blah blah
      blah blah blah blah blah blah blah blah blah
      blah blah blah blah blah blah blah blah blah
      blah blah blah blah blah blah blah blah blah
};
\end{tikzpicture}

I just wanted to augment Tobi's answer regarding execute at begin/end node, and put that part into an own style with an argument for the width.

\tikzset{
    max width/.style args={#1}{
        execute at begin node={\begin{varwidth}{#1}},
        execute at end node={\end{varwidth}}
    }
}
\begin{tikzpicture}
    \node[max width=3cm] {this node is maximum 3cm wide although its text is wider};
    \node[max width=10cm] {shrinks to textwidth};
\end{tikzpicture}

Dont forget to load the varwidth package beforehand: \usepackage{varwidth}