Bounding box and umlaut issue in text decoration

Question

In the above TikZ picture, how can I

• extend the bounding box to include the text decoration without manual tinkering (the only solution I was able to find is fiddle with the border or margin class options, see here)?
• avoid the error Package inputenc Error: Unicode char from the sharp s "ß" on line 24?

Code

\documentclass[tikz,svgnames]{standalone}

\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}

\usetikzlibrary{positioning,decorations.text}

\renewcommand{\familydefault}{\sfdefault}

\tikzset{
    entity/.style={fill=#1!70,text=white},
    action/.style={->,postaction={decorate,decoration={raise=2pt,text along path,text align=center,text={|\scriptsize|#1}}}}
}

\begin{document}
\begin{tikzpicture}

    % Entities
    \node[entity=DarkGreen] (A) {A};

    \node[entity=DarkBlue,right=2 of A] (B) {B};

    % Actions
    \draw[action=schließen] (A) to [bend left]  (B);
\end{tikzpicture}    
\end{document}

Answer 1

This adds an invisible line above the decorations. If you make the line visible, you will see that the bounding box is slightly larger than needed.

I played around with all the suggestions regarding \scriptsize and concluded that the OP solution is best.

\documentclass[tikz,svgnames]{standalone}

\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}

\usetikzlibrary{positioning,decorations.text,calc}

\renewcommand{\familydefault}{\sfdefault}

\tikzset{
    entity/.style={fill=#1!70,text=white},
    action/.style={->,postaction={decorate,decoration={raise=2pt,text along path,
      text align=center,text={|\scriptsize|#1}
}}}}

\newlength{\mybuffer}
\sbox0{\scriptsize\strut}
\setlength{\mybuffer}{\dimexpr 2pt+\ht0+\dp0}

\begin{document}

\begin{tikzpicture}
    % Entities
    \node[entity=DarkGreen] (A) {A};

    \node[entity=DarkBlue,right=2 of A] (B) {B};

    % Actions
    \draw[action={schlie{ß}en}] (A) to [bend left]  (B);
    \path ($(A)+(0pt,\mybuffer)$) to [bend left] ($(B)+(0pt,\mybuffer)$);
    %\draw[red] ($(A)+(0pt,\mybuffer)$) to [bend left] ($(B)+(0pt,\mybuffer)$);
\end{tikzpicture}    
\end{document}