LaTeX PGF Export: How to change size globally for all .tex-Files

[Pooya <input pooya de>]

Hi there,

I tried to define the scale-variable in a PGF-exported file in a global LaTeX file by the command 
corresponding to that in the .tex-File:

\newlength{\du}
\setlength{\du}{19\unitlength}

I was thinking, the new value 19 would overwrite all other values (15) written in all of my exported 
tex-files.
But it doesn´t :-)

How to redefine this properly?
Thanks in advance
Cheers.