You can decide to scale
nodes for example by setting:
\tikzset{every node/.append style={scale=0.6}}
and then locally redefine the scaling just in the node you are interested in.
For example:
\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage{tikz}
\usetikzlibrary{mindmap}
\tikzset{level 1 concept/.append style={font=\sf, sibling angle=90,level distance = 25mm}}
\tikzset{level 2 concept/.append style={font=\sf, sibling angle=45,level distance = 15mm}}
\tikzset{level 3 concept/.append style={font=\sf, sibling angle=45,level distance = 15mm}}
\tikzset{every node/.append style={scale=0.6}}
\begin{document}
\begin{tikzpicture}[mindmap, concept color=blue, font=\sf\bf, text=white]
\node[concept]{Root Concept}[clockwise from=315]
child[concept color=orange] {node[concept] (c1){Child 1}
child [concept color=orange] {node [concept](c11){Child 1-1}}
child [concept color=orange] {node [concept](c12){Child 1-2}}
child [concept color=orange] {node [concept](c13){Child 1-3}}
}
child [concept color=orange] {node [concept](c2){Child 2}
child [concept color=orange] {node [concept,scale=0.4](c21){Child 2-1}}
child [concept color=orange] {node [concept](c22){Child 2-2}}
child [concept color=orange] {node [concept](c22){Child 1-3}}
};
\end{tikzpicture}
\end{document}
will give you:

Notice the local redefinition in:
child [concept color=orange] {node [concept,scale=0.4](c21){Child 2-1}}
[concept, minimum size=3cm]
but for maximum. It is not related to the text therein. – Googlebot Jun 2 '12 at 9:45