I have used a combination of the MWE of @Bernard and the macro of @Steven B. Segletes here to obtain different dimension of a \bullet:
Is there a black 'dot' symbol that I can use?:
\documentclass[12pt]{article}
\usepackage[utf8]{inputenc}
\newcommand{\comment}[1]{}
\usepackage{graphicx,scalerel}
\newcommand\fsbullet[1][.8]{\mathbin{\ThisStyle{\vcenter{\hbox{%
\scalebox{#1}{$\SavedStyle\bullet$}}}}}%
}
\newcommand\ssbullet[1][.6]{\mathbin{\ThisStyle{\vcenter{\hbox{%
\scalebox{#1}{$\SavedStyle\bullet$}}}}}%
}
\newcommand\tsbullet[1][.4]{\mathbin{\ThisStyle{\vcenter{\hbox{%
\scalebox{#1}{$\SavedStyle\bullet$}}}}}%
}
\newcommand\fosbullet[1][.3]{\mathbin{\ThisStyle{\vcenter{\hbox{%
\scalebox{#1}{$\SavedStyle\bullet$}}}}}%
}
\renewcommand{\labelitemi}{\(\fsbullet\)}
\renewcommand{\labelitemii}{\(\ssbullet\)}
\renewcommand{\labelitemiii}{\(\tsbullet\)}
\renewcommand{\labelitemiv}{\(\fosbullet\)}%
\begin{document}
\begin{itemize}
\item First bullet \verb|medium size:\fsbullet|
\begin{itemize}
\item Second bullet \verb|small size:\ssbullet|
\begin{itemize}
\item Third bullet \verb|tiny size:\tsbullet|
\begin{itemize}
\item First bullet \verb|footnote size:\fosbullet|
\end{itemize}
\end{itemize}
\end{itemize}
\end{itemize}
\end{document}

In addition, for example, the parameter a give you the dimension/size of the bullet: \ssbullet[1][a].
\circ\renewcommand{\labelitemiv}{\scalebox{1.3}{\(\circ\)}}or scaling down\bigcirc\renewcommand{\labelitemiv}{\scalebox{.7}{\(\bigcirc\)}}. Note that\scaleboxrequires thegraphicxpackage. – Sandy G Mar 25 '20 at 21:13