Yes, if you by size mean bounding box. PGF does a good job determining
a bounding box automatically, but if you need to set a bounding box
explicitly you can apply the 'use as bounding box' option to a path.
PGF will then use the path as a bounding box. This is described in the
section "Establishing a Bounding Box" of the PGF manual.
Here is a simple example:
...
\begin{tikzpicture}
\path[use as bounding box] (0,0) rectangle (10,10);
\draw (1,1) -- (3,3);
...
\end{tikzpicture}
...
- Kjell Magne Fauske