May be a round fitbox (from tcolorbox) can help you. You just define box size and tcolorbox fits the text inside it. Only texts larger than provided space are reduced.
\documentclass{article}
\usepackage[most]{tcolorbox}
\usepackage{lmodern}
\newtcboxfit{\myfitbox}{%
width = 3cm,
square,
circular arc,
halign = center,
valign = center,
nobeforeafter
}
\begin{document}
\myfitbox{This is a long text inside a circular tcolorbox}
%
\myfitbox{This is an ever longer text inside a circular tcolorbox}
%
\myfitbox{This is a much more longer, longer and longer text inside a circular tcolorbox}
%
\myfitbox{This is a much more longer, longer and longer text inside a circular tcolorbox. This is a much more longer, longer and longer text inside a circular tcolorbox}
\end{document}

width("some text here"). Sort of related: https://tex.stackexchange.com/questions/373941/flow-chart-with-defined-width-for-the-boxes/374271#374271 – Torbjørn T. Aug 04 '17 at 11:58