Something as simple as adding a \path between the two corners you want to be the bounding box will do the trick as long as the rest of your image is contained within that.
A better approach may be to explicitly fix the size of the bounding box with \useasboundingbox macro, which can give explicit coordinates, or coordinates relative to another tikzpicture as described in this answer. This will prevent the bounding box from expanding if something is placed outside the bounding box, which may or may not be desirable.
\documentclass{article}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
\node at (2,2) {Foo};
\draw (current bounding box.north east) -- (current bounding box.north west) -- (current bounding box.south west) -- (current bounding box.south east) -- cycle;
\end{tikzpicture}
\begin{tikzpicture}
\path (0,0) -- (5,5);
\node at (2,2) {Foo};
\draw (current bounding box.north east) -- (current bounding box.north west) -- (current bounding box.south west) -- (current bounding box.south east) -- cycle;
\end{tikzpicture}
\begin{tikzpicture}
\useasboundingbox (0,0) rectangle (5,5);
\node at (2,2) {Foo};
\node at (7,2) {Bar};
\draw (current bounding box.north east) -- (current bounding box.north west) -- (current bounding box.south west) -- (current bounding box.south east) -- cycle;
\end{tikzpicture}
\end{document}

\path (0.1,0.1) rectangle (2.1,2.12);. As long as this rectangle is bigger then all your components, it will determine the size of the bounding box. – samcarter_is_at_topanswers.xyz Nov 05 '17 at 12:11