Change node size of a tree

Question

I have this tree:

That was created with this code:
begin{figure}[h]
        centering
        begin{tikzpicture}
            node[circle,draw]{}
            child{
                node[circle,draw]{a}
            }
            child[missing]{};
        end{tikzpicture}
    end{figure}

How do I increase the size of the root circle so that, even when empty, it looks like the bottom circle? Or is there a better way to create these trees?

Zarko · Accepted Answer

With use of the forest package:
documentclass[margin=3mm]{standalone}
usepackage{forest}

begin{document}
    begin{forest}
for tree = {circle,
            draw,
            minimum size=1.1em,
            inner sep=2pt,
            font=small,
    l sep=9mm,
    s sep=6mm
            }
[
    [a]
    [, coordinate, no edge]
]   
    end{forest}
end{document}

Sebastiano · Answer

Welcome to the TeX.SE community. I will use, as a fast solution an hack, putting into node[circle,draw]{...} a phantom character: phantom{small a}.

documentclass[a4paper,12pt]{article}
usepackage{tikz}
usepackage{graphicx}

begin{document}
begin{figure}[h]
        centering
        begin{tikzpicture}
            node[circle,draw]{phantom{small a}}
            child{
                node[circle,draw]{a}
            }
            child[missing]{};
        end{tikzpicture}
    end{figure}
end{document}

Alan Munn · Answer

You can simply define the size of the tree nodes to have a minimum. Since it's likely that the majority of your nodes will also be drawn as circles, and you may have more than one tree that looks like this,  it makes sense to create a global style for re-use including all of those parameters.
If you're drawing a lot of trees, I would strongly recommend switching to forest. The base TikZ tree drawing tools are really not very usable: their syntax is very clunky and they do no automatic node packing.
documentclass[a4paper,12pt]{article}
usepackage{tikz}
tikzset{my tree/.style={every node/.style={draw,circle,minimum size=1.5em}}}begin{document}
        begin{tikzpicture}[my tree]
            node{}
            child{
                node{a}
            }
            child[missing]{};
        end{tikzpicture}
end{document}

Change node size of a tree

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