I am using
pdflatex and just want to know how to change the footnote line (thickness, length).
Besides that: How can I set the distance between text-lines in the text.
The footnote rule should take no space on the page. That's the purpose of
\hrule width 2in
First of all TeX backs up vertically by 3pt, then prints the rule (which is by default 0.4pt thick) and then skips by 2.6pt: the total is zero.
So a correct redefinition would be
\hrule width \textwidth height 1pt
\textwidth one can also put an explicit dimension. The rule height added to the positive kern after the rule should give the negative of the back up amount preceding the rule.
One should note that the amount of skipping down gives the separation between the rule and the footnote text. With this definition the rule will be at 2pt + height of strut from the baseline of the footnote's first line. Before the rule there's a vertical skip measured by
\skip\footins, whose default value is 9pt plus 4pt minus 2pt. With the 3pt back up, the space between the text and the footnote rule will be at least 4pt and at most 10pt.
Jan Tschichold frowned upon footnote rules: he said that there should be none at all or it must occupy the entire text width.
You can redefine
\footnoterule to do what you want. For instance, here's a really ugly footnote rule:
The first command of the
\rule command is how long it is, the second how thick.
Given that the footnote rule should not actually take up any space on the page, egreg's answer should be preferred to this one.