How to scale a tikzpicture including texts?

There are a few things that scaling doesn't affect; the most noticeable are node sizes and line widths. In a simple picture, it isn't hard to adjust the line width accordingly but the nodes can be difficult. It is possible to force a node to be scaled: put the scale option directly in the node's attributes. Thus node[above,scale=0.6] at (8,11) {true}; would scale the node. This is a bit annoying to put on every node, so there's an every node style that can be used to do this. Thus:

begin{tikzpicture}[thick,scale=0.6, every node/.style={scale=0.6}]

Even so, you'd still have to remember to change two things each time here if you wanted to change the scale factor. Fortunately, there's a key transform shape which means that the current transformation is applied to the node. The danger with using this is that this will also apply any rotations that happen to be in effect to the node (normally only translations are applied). If you don't have any rotations, then:

begin{tikzpicture}[thick,scale=0.6, every node/.style={transform shape}]

will do just fine.

If you do have or worry about those rotations (or for anyone else interested) it would be simple to set a global scale key to fix this:

tikzset{global scale/.style={
    scale=#1,
    every node/.style={scale=#1}
  }
}

Back to the "normal" solutions. Here's the various solutions:

documentclass{standalone}
usepackage{tikz}

begin{document}

framebox{begin{tikzpicture}[thick]
draw [dashed] (1,12) -- (11,12);
node[above] at (4,11) {1}; node[above] at (8,11) {true};
end{tikzpicture}}

framebox{begin{tikzpicture}[thick, scale=0.6]
draw [dashed] (1,12) -- (11,12);
node[above] at (4,11) {1}; node[above] at (8,11) {true};
end{tikzpicture}}

framebox{begin{tikzpicture}[thick, transform canvas={scale=0.6}]
draw [dashed] (1,12) -- (11,12);
node[above] at (4,11) {1}; node[above] at (8,11) {true};
end{tikzpicture}}

framebox{begin{tikzpicture}[thick,scale=0.6, every node/.style={scale=0.6}]
draw [dashed] (1,12) -- (11,12);
node[above] at (4,11) {1}; node[above] at (8,11) {true};
end{tikzpicture}}

framebox{begin{tikzpicture}[thick,scale=0.6, every node/.style={transform shape}]
draw [dashed] (1,12) -- (11,12);
node[above] at (4,11) {1}; node[above] at (8,11) {true};
end{tikzpicture}}

end{document}

I put the frameboxs in because if you count carefully, you'll see that the third example isn't there! In fact, it ended up somewhere at the top of the page, outside what the standalone package thought the page was. So it got clipped out.