Hi Andreas,
1. Where can I find the chktexrc-file?
That should live inside of the directory
/usr/local/texlive/2011/texmf/chktex
if you're using texlive 2011, and similarly for 2012 and so forth.
2. And how can I modify it to make ChkTeX ignoring
named above warning?
That file looks to be extensively commented. Search for "length of"
and it'll take you to the relevant section.
Cheers,
Paul