Change environment's width

Question

I'm writing my homework and I would like to change the environment's dimensions of

\documentclass[10pt,spanish]{article}

% INICIO PAQUETES


\usepackage{amsmath}
\usepackage{graphicx}
\usepackage[strict]{changepage} % para cambiar dimensiones (recomendado en la web)
\usepackage{fancyhdr}
\usepackage{mathrsfs}
\usepackage[spanish]{babel}
\usepackage[utf8]{inputenc}
\usepackage{color}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{amsthm} % gracias a este paquete podemos enlistar los teoremas
\usepackage{hyperref} % permite agregar links

\let\UPproof\proof\def\proof{\small\UPproof}

\newtheorem{teo}{Teorema}[section]
\newtheorem{pro}{Proposici\'on}[section]
\newtheorem{cor}{Corolario}[section]
\newtheorem{lem}{Lema}[section]
\newtheorem{mydef}{Definici\'on}[section]

\textwidth 19cm
\topmargin -2cm
\textheight 23cm
\oddsidemargin -1,2cm
\parindent 0em
\parskip 2ex


\title{Un título}
\date{Una fecha}
\begin{pro} This tense is false.
\end{pro}
\begin{proof}
Because yes, yes.
\end{proof}
\end{document}

I want to make it (the "box" where the proof is being written) an x % smaller than the original "box" where the proposition is, but I don't know how to do it so if someone can help me, I'd be grateful.

Answer 1

Wrap the proof environment from amsthm inside an adjustwidth environment (from changepage) with suitable margin adjustments:

\documentclass{article}
\usepackage{amsthm,letltxmacro,changepage}
\usepackage{lipsum}% Just for this example

\LetLtxMacro\oldproof\proof
\let\endoldproof\endproof
\renewenvironment{proof}[1][\proofname]
  {\begin{adjustwidth}{2em}{2em}
   \oldproof[#1]}
  {\endoldproof
   \end{adjustwidth}}
\begin{document}

\lipsum[1]

\begin{proof}
\lipsum*[2]
\end{proof}

\lipsum[3]

\end{document}

The margin adjustments above are both 2em inward from the text block margins.