circuitikz: Controlling the size of poles

As described in the manual, the size of the poles is defined by the nodes width key, which by default is 0.04. This number is a fraction of some "basic length". Change it with e.g. ctikzset{nodes width=0.2}.

Below I used labels instead of adding extra nodes, but that's merely a suggestion.

documentclass[12pt,a4paper]{article}
usepackage{circuitikz}
ctikzset{nodes width=0.2} % default = 0.04
begin{document}
    begin{tikzpicture}
                    node (a) at (0,0) [ocirc, label=left:$a$] {};
                    node (c) at (1.5,0) [ocirc, label=left:$c$] {};
                    node (b) at (3,0) [ocirc, label=right:$b$] {};
                    node (d) at (1.5,2) [ocirc, label=above:$d$] {};
                    node (e) at (1.5,-2)[ocirc, label=below:$e$] {};
                    
                    
                    path 
                    (a) edge           (d)
                    edge           (e)
                    (b) edge           (d)
                    edge           (e)
                    (d) edge           (c)
                    (c) edge           (e); 
    end{tikzpicture}
end{document}

A couple of examples for how to apply this to just one diagram:

documentclass{article}
usepackage{circuitikz}
begin{document}
    begin{tikzpicture}
         node (a) at (0,0) [ocirc, label=left:$a$] {};
    end{tikzpicture}
    
    begin{tikzpicture}
        ctikzset{nodes width=0.2} 
         node (a) at (0,0) [ocirc, label=left:$a$] {};
    end{tikzpicture}
    
    begin{tikzpicture}
         node (a) at (0,0) [ocirc, label=left:$a$] {};
    end{tikzpicture}
    
    begin{tikzpicture}[/tikz/circuitikz/nodes width=0.2]
         node (a) at (0,0) [ocirc, label=left:$a$] {};
    end{tikzpicture}

   begin{tikzpicture}
         node (a) at (0,0) [ocirc, label=left:$a$] {};
    end{tikzpicture}
    
end{document}