The above works perfectly on Safari 6.0.x with the following tweaking:
1. The file in question was changed of location when safari was upgraded. It's new path is:
/System/Library/StagedFrameworks/Safari/Safari.framework/Versions/A/Resources/R eader.html
2. Adding the changes just before the style declaration closes works because it will overwrite the defaults that are set previously in the file but more elegantly you can actually find those tags mentioned and manually replace the values yourself.
3. It won't suffice to refresh the window for the changes to take effect. You want to kill the thread and reopen it. You don't have to quit and reopen safari, it's enough to close the tab and open it again and you'll be set đ
ENJOY!
PS: I actually grew it 600px and it looks beautiful in my 27 inch screen