Choose size of text inside a framed box and align it with the text width

Question

I am using a framed box to put forward some texts. I have two problems:

• First, I would like to minimize the text size; as we can do for the legend of a figure: \captionsetup{width=0.95\textwidth, font=small}.

• Secondly, I have a problem with my alignment as regards the text. The framed boxes stay on the right at each page. I suspect a problem with all the others packages. That's why they are all here. Sorry for the long list.

Here is the code I use:

\documentclass{article}

% I put all packages to prevent any conflict

\PassOptionsToPackage{table,xcdraw}{xcolor}
\documentclass[12pt, french, onehalfspacing, headsepline, table,]{MastersDoctoralThesis}

\usepackage[utf8]{inputenc}
\usepackage[backend=bibtex,style=ieee,natbib=true, doi = false, url=false, arxiv=false]{biblatex}
\addbibresource{library.bib}
\usepackage[autostyle=true]{csquotes}
\usepackage{microtype, mparhack, relsize}
\usepackage{amsmath}
\usepackage[bottom]{footmisc}
\usepackage{capt-of}
\usepackage{caption}
\captionsetup{width=0.95\textwidth, font=small}
\usepackage{subcaption}
\usepackage{float}
\usepackage{graphicx}
\usepackage{amssymb}
\usepackage{minitoc}
\usepackage{enumitem}
\setlist[itemize]{label=\textbullet}
\usepackage{mathtools, nccmath}

% End of the long list


%Needed for the framed box: 

\usepackage[framemethod=TikZ]{mdframed}
\usepackage{lipsum}
\mdfdefinestyle{MyFrame}{%
    linecolor=blue,
    outerlinewidth=2pt,
    roundcorner=20pt,
    innertopmargin=\baselineskip,
    innerbottommargin=\baselineskip,
    innerrightmargin=20pt,
    innerleftmargin=20pt,
    backgroundcolor=gray!50!white}

\begin{document}
\lipsum[1-3]
\parbox{\textwidth}{\begin{mdframed}[style=MyFrame]
\lipsum[1]
\begin{equation}
 f(x) = \sin(x)
\end{equation}
\lipsum[2]
\end{mdframed}}
\end{document}