The line widths thin, thick, etc. are defined liked this (taken from here):
\tikzset{
ultra thin/.style= {line width=0.1pt},
very thin/.style= {line width=0.2pt},
thin/.style= {line width=0.4pt},
semithick/.style= {line width=0.6pt},
thick/.style= {line width=0.8pt},
very thick/.style= {line width=1.2pt},
ultra thick/.style={line width=1.6pt}
}
So I would suggest to replace this definition with every line widths given as a multiple of some length variable (say \mylinewidth). You can then change the value of \mylinewidth globally or locally, and the lines with these options will adjust their widths.
Example (note the comment below):
\documentclass{article}
\usepackage{tikz}
\newlength\mylinewidth
\setlength\mylinewidth{0.4pt}
\tikzset{
ultra thin/.style= {line width=0.25\mylinewidth},
very thin/.style= {line width=0.5\mylinewidth},
thin/.style= {line width=\mylinewidth},
semithick/.style= {line width=1.5\mylinewidth},
thick/.style= {line width=2\mylinewidth},
very thick/.style= {line width=3\mylinewidth},
ultra thick/.style={line width=4\mylinewidth},
every picture/.style={semithick}
}
\begin{document}
\begin{tikzpicture}
\draw[very thin] (0,0) -- ++(2,0);
\draw (0,1) -- ++(2,0);
\draw[very thick] (0,2) -- ++(2,0);
\end{tikzpicture}
\hspace{1cm}
\setlength\mylinewidth{1pt}
\begin{tikzpicture}
\draw[very thin] (0,0) -- ++(2,0);
\draw (0,1) -- ++(2,0);
\draw[very thick] (0,2) -- ++(2,0);
\end{tikzpicture}
\end{document}

Note: In this example, I used semithick as the default line width because it appeared visually right to me. You can of course use the standard default which is every picture/.style={thin}.
thin,thickand alike, but wish for a one-line way of changing all of these definitions. So yes, the MWE is needed. – Tiuri Aug 31 '17 at 10:18