Q:

Remove "white parts" of tikz picture

I have a tikz picture, which consists of a visible part on the left side and a white part on the right side (for some constructive reasons). Now I would like to redefine the size so that the new picture just consists of the left part (without scaling).
How do you do that?

A:

You have to explicitly set the bounding box of the picture. TikZ provides the use as bounding box option to do that. For example
\documentclass{report}
\usepackage{tikz}

\begin{document}
a
\begin{tikzpicture}
    \path[use as bounding box, draw] (0,0) rectangle (2,2); % The draw option is given just for demonstration.
    \draw (0,0) circle [radius=1cm];
\end{tikzpicture}
b
\end{document}

If you want to shrink an already established bounding box, you have to reset it with \pgfresetboundingbox first:
\documentclass{report}
\usepackage{tikz}

\begin{document}
a
\begin{tikzpicture}
    \draw (0,0) circle [radius=1cm];
    \pgfresetboundingbox
    \path[use as bounding box, draw] (0,0) rectangle (2,2);
\end{tikzpicture}
b
\end{document}

Other useful options for modifying the bounding box are the trim left and trim right options to tikzpicture. Section 15.7 “Establishing a Bounding Box” of the TikZ manual (v2.10) has all the details.

A:

You could use the \clip command to restrict your figure to a certain area (for further details, see Secion 15.8 Clipping and Fading (Soft Clipping) of the pgfmanual). A little example:
\documentclass{article}
\usepackage{tikz} 

\begin{document}

text \begin{tikzpicture}[line width=5pt]
\draw (0,0) -- (1,0) -- (1,1) -- cycle;
\draw (2,0) -- (3,0) -- (3,1) -- cycle;
\end{tikzpicture} text

text \begin{tikzpicture}[line width=5pt]
\clip (-0.2,-0.1) rectangle (1.2,1.2);
\draw (0,0) -- (1,0) -- (1,1) -- cycle;
\draw (2,0) -- (3,0) -- (3,1) -- cycle;
\end{tikzpicture} text

\end{document}

