How do I set a really exact radius for a node?

Creating an energy level diagram, I used this answer to make the node circles equal radius: How to set exact radius for a node?

\documentclass{report}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
\node[circle,draw,blue,text width=0.3cm,text height=0.3cm] at (5,9) {$\uparrow$};% 4d
\node[circle,draw,blue,text width=0.3cm,text height=0.3cm] at (6,9) {$\uparrow$};% 4d
\node[circle,draw,blue,text width=0.3cm,text height=0.3cm] at (7,9) {};% 4d
\node[circle,draw,blue,text width=0.3cm,text height=0.3cm] at (8,9) {\phantom{6}};% 4d
\node[circle,draw,blue,text width=0.3cm,text height=0.3cm] at (9,9) {\quad};% 4d
\end{tikzpicture}
\end{document}

But the empty nodes are still ever so slightly smaller than the filled ones! Tried including phantom or quad to no avail.

What am I missing?

You don't have to use phantoms, etc, to control the size of the circles. Just use the option for all circles minimum size=<length>, choosing a large-enough size, for example minimum size=0.7cm. Setting things in a cir/.style makes the code more elegant.

\documentclass{report}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}[cir/.style={circle,draw,blue,minimum size=0.7cm}]
\node[cir] at (5,9) {$\uparrow$};% 4d
\node[cir] at (6,9) {$\uparrow$};% 4d
\node[cir] at (7,9) {};% 4d
\node[cir] at (8,9) {};% 4d
\node[cir] at (9,9) {};% 4d
\end{tikzpicture}
\end{document} • I'd reduce inner sep to remove some of the space. – gernot Jan 23 '17 at 1:32