documentclass{article}
usepackage{tikz}
begin{document}

section*{Without Scaling}
begin{tikzpicture}
node at (0, 0) [rectangle,draw,minimum size=5cm] {A};
draw (-2.5, 1) -- (2.5, 1) node[midway,below] {B};
end{tikzpicture}

section*{With Scaling}
begin{tikzpicture}[scale=0.5]
node at (0, 0) [rectangle,draw,minimum size=5cm] {A};
draw (-2.5, 1) -- (2.5, 1) node[midway,below] {B};
end{tikzpicture}

end{document}