Another possibility:Warning: use at your own risk, nesting tikzpictures not recommended
You could put the tikzpicture into a node, and scale the node
\tikz \node [scale=0.5, inner sep=0] { \begin{tikzpicture} \draw [line width=10pt] (0,0) rectangle (1,1); \node at (0.5,0.5) {abc}; \end{tikzpicture} }; 
(left: without scaling, right: with scaling)
Another possibility: put the tikzpicture into a node, and scale the node
\tikz \node [scale=0.5, inner sep=0] { \begin{tikzpicture} \draw [line width=10pt] (0,0) rectangle (1,1); \node at (0.5,0.5) {abc}; \end{tikzpicture} };
(left: without scaling, right: with scaling)
Warning: use at your own risk, nesting tikzpictures not recommended
You could put the tikzpicture into a node, and scale the node
\tikz \node [scale=0.5, inner sep=0] { \begin{tikzpicture} \draw [line width=10pt] (0,0) rectangle (1,1); \node at (0.5,0.5) {abc}; \end{tikzpicture} };
(left: without scaling, right: with scaling)
Another possibility: put the tikzpicture into a node, and scale the node
\tikz \node [scale=0.5, inner sep=0] { \begin{tikzpicture} \draw [line width=10pt] (0,0) rectangle (1,1); \node at (0.5,0.5) {abc}; \end{tikzpicture} };
(left: without scaling, right: with scaling)