How do I make the size of the node larger?
\begin{tikzpicture}
\node [left,ultra thick] at (0,6) {$V_i$};
\node [left,ultra thick] at (0,3) {$V_f$};
\end{tikzpicture}
How do I make the size of the node larger?
\begin{tikzpicture}
\node [left,ultra thick] at (0,6) {$V_i$};
\node [left,ultra thick] at (0,3) {$V_f$};
\end{tikzpicture}
Using: minimum size=4cm you can increase the size of the node:
\documentclass[a4paper,12pt]{article}
\usepackage{tikz,mathtools}
\begin{document}
\begin{tikzpicture}
\node [left,minimum size=4cm] at (0,6) {$V_i$};
\node [left,minimum size=4cm] at (0,3) {$V_f$};
\end{tikzpicture}
\end{document}
If the question is increase the size of the character, then you can add \large, \Huge, etc., as from this code:
\documentclass[a4paper,12pt]{article}
\usepackage{tikz,mathtools}
\begin{document}
\begin{tikzpicture}
\node [left,minimum size=4cm] at (0,6) {\large{$V_i$}};
\node [left,minimum size=4cm] at (0,3) {\Huge{$V_f$}};
\end{tikzpicture}
\end{document}