根据论文格式，我希望章节标题位于中心，还希望定义章节、部分、小节的字体大小

Question

\documentclass[a4paper,12pt, twoside]{report}
\usepackage[margin=1in]{geometry}
\usepackage{setspace}
\onehalfspacing
\usepackage{graphicx}
\renewcommand{\baselinestretch}{1.5} 
\begin{document}
% Content starts
\pagenumbering{roman}
\chapter*{Declaration}
declaration goes here

\chapter*{Acknowledgements}
\begin{abstract}
    I hereby certify that the work which is presented in dissertation entitled,  in partial fulfillment of the....
\end{abstract}
Acknowledgements
\tableofcontents
\listoffigures
\listoftables

\input{Abbreviations}

diabetes mellitus etc etc
\pagenumbering{arabic}
\input{Ch1_Intro}
\input{Ch2_Literature}
\input{Ch3_Prob.st.}
\input{Ch4_Methods}
\input{Ch5_Results}
\input{Ch6_Future}
\end{document}