Another option is to use the tcolorbox package which easily gives you frames in which you can control the individual padding to the left, to the right, to the top and bottom, among many other attributes. A little example:
\documentclass{article}
\usepackage[most]{tcolorbox}
\newtcolorbox{myframe}[1][]{
enhanced,
arc=0pt,
outer arc=0pt,
colback=white,
boxrule=0.8pt,
#1
}
\title{The Title}
\author{The Author}
\begin{document}
\begin{myframe}
I certify that the work presented in the dissertation is my own unless referenced.
\end{myframe}
\begin{myframe}[width=30em]
I certify that the work presented in the dissertation is my own unless referenced.
\end{myframe}
\begin{myframe}[width=30em,top=10pt,bottom=10pt,left=20pt,right=20pt]
I certify that the work presented in the dissertation is my own unless referenced.
\end{myframe}
\begin{myframe}[width=30em,top=20pt,bottom=20pt,left=20pt,right=20pt,arc=10pt,auto outer arc]
I certify that the work presented in the dissertation is my own unless referenced.
\end{myframe}
\end{document}
