Q:

change fontsize within tikzpicture

i made some tikzpicture and tested them outside of the main document cause then there's less compiling duration.  
However i alway used \documentclass{article}.  
I set the fonts with  \begin{tikzpicture}[auto,  font = {\sf \myfont}, thick]. 
For \myfont i used \newcommand{\myfont}{\footnotesize}. 
After creating the tikzpicture i wanted to add the tikzpicture with \input{mytikzpicture.tex} in to my main document.  
But somehow the change of the fontsize i set with \begin{tikzpicture}[auto,  font = {\sf \myfont}, thick] has no effect now. 
In my main document i set the font size using this code:
\documentclass[
    pdftex,
    a4paper,
    11pt,
    oneside,
    fleqn,
    listof=totoc,
    headlines=2.1,
    headsepline,
    numbers=noenddot,
    hyphens,
]{scrreprt}

What i already tried:

\tikzset{font={\fontsize{9pt}{12}\selectfont}}
I wanted  to use this to decrease the font size in a tikzpicture only.  I got this solution from searching the internet (Change fontsize in TikZ figure).  Unfortunately it had no effect on the fontsize in the tikzpicture at all. 
changing the fontsize using kind of scoping (also no effect): found at
this thread
{\footnotesize % change the font size
  \begin{tikzpicture}[auto,  font = {\sf \myfont}, thick]
   ...
  \end{tikzpicture}
} % end of scope of \footnotesize macro

What's the reason it doesn't work and what can i change? 
If i didn't provide all necessary information to solve the problem feel free to tell me. 
edit1:
here a minimum example of my problem. 
the tex file i use for developing my tikzpictures:
\documentclass{article}
\usepackage{amsmath}
\usepackage[latin1]{inputenc}
\usepackage{tikz}
\usepackage{textgreek}
\usetikzlibrary{shapes,arrows}
\usetikzlibrary{shapes.multipart}
\usetikzlibrary{positioning}
\usetikzlibrary{arrows.meta, calc, positioning, shapes, matrix}
\usetikzlibrary{arrows}
\usetikzlibrary{calc}

\newcommand{\myfont}{\footnotesize}
\tikzstyle{block} = [draw,
rounded corners]

\begin{document}

    \begin{tikzpicture}[auto,  font = {\myfont}, thick]
    % Hilfslinien
    \draw[help lines] (-5,-5) grid (5,5);
    \node[block, align=center] (isolation) {hello};
    \end{tikzpicture}

\end{document}

minimum example if i put my developed stuff into the main document:
\documentclass[
pdftex,
a4paper,
11pt,
oneside,
fleqn,
listof=totoc,
headlines=2.1,
headsepline,
numbers=noenddot,
hyphens,
]{scrreprt}
\usepackage{amsmath}
\usepackage[latin1]{inputenc}
\usepackage{tikz}
\usepackage{textgreek}
\usetikzlibrary{shapes,arrows}
\usetikzlibrary{shapes.multipart}
\usetikzlibrary{positioning}
\usetikzlibrary{arrows.meta, calc, positioning, shapes, matrix}
\usetikzlibrary{arrows}
\usetikzlibrary{calc}

\newcommand{\myfont}{\footnotesize}
\tikzstyle{block} = [draw,
rounded corners]

\begin{document}

    \begin{tikzpicture}[auto,  font = {\myfont}, thick]
    % Hilfslinien
    \draw[help lines] (-5,-5) grid (5,5);

    \node[block, align=center] (isolation) {hello};
    \end{tikzpicture}

\end{document}

Outputs:

The left side shows the output of my 'devolopment' file.  The right is the output of the main document.  
As you can see if i put it into the main document the fonts are slightly bigger.  This is enough to distort all my tikzpictures. 
Thank in advance

A:

The default font size for KOMA-Script classes is 11pt, but for standard classes it is 10pt. 
Maybe you can change the font size for all tikzpictures to 10pt using
\usepackage{xpatch}
\xpretocmd\tikzpicture{\KOMAoptions{fontsize=10pt}}{}{\PatchFailed}

Code:
\documentclass[
  fontsize=11pt% 11pt is default for KOMA-Script classes
]{scrreprt}
\usepackage{tikz}
\newcommand{\myfont}{\sffamily\footnotesize}
\tikzset{block/.style = {draw,rounded corners}}

\usepackage{xpatch}
\xpretocmd\tikzpicture{\KOMAoptions{fontsize=10pt}}{}{\PatchFailed}

\begin{document}
\begin{tikzpicture}[auto,font = {\myfont}, thick]
% help lines
\draw[help lines,lightgray!50] (-5,-5) grid[step=2mm] (5,5);
\draw[help lines] (-5,-5) grid (5,5);

\node[block] (isolation) {hello};
\end{tikzpicture}
\end{document}

Example with article:
\documentclass{article}% default font size is 10pt
\usepackage{tikz}
\newcommand{\myfont}{\sffamily\footnotesize}
\tikzset{block/.style = {draw,rounded corners}}

\begin{document}
\begin{tikzpicture}[auto,font = {\myfont}, thick]
% help lines
\draw[help lines,lightgray!50] (-5,-5) grid[step=2mm] (5,5);
\draw[help lines] (-5,-5) grid (5,5);

\node[block] (isolation) {hello};
\end{tikzpicture}
\end{document}

Result:

