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.
\let\UPproof\proof\def\proof{\small\UPproof}
placed in the preamble should do. It would certainly be easier to answer your question if you provided a minimal working example (MWE). – yo' Oct 17 '15 at 19:45proof
environment takes up with respect to the\textwidth
or text block containing other text, correct – Werner Oct 17 '15 at 19:56