Funes el memorioso

\documentclass{minimal}
% for smallper page
\usepackage[papersize={2.6in, 1.2in}, text={2.6in, 1.2in}]{geometry}
% for fancy math
\usepackage{amsmath}

% Greek letter lowercase boldface
\newcommand{\alphabf}{\ensuremath{\boldsymbol{\alpha}}}
\newcommand{\betabf}{\ensuremath{\boldsymbol{\beta}}}
\newcommand{\gammabf}{\ensuremath{\boldsymbol{\gamma}}}
\newcommand{\deltabf}{\ensuremath{\boldsymbol{\delta}}}
\newcommand{\epsbf}{\ensuremath{\boldsymbol{\epsilon}}}
\newcommand{\epsilonbf}{\ensuremath{\boldsymbol{\epsilon}}}
\newcommand{\zetabf}{\ensuremath{\boldsymbol{\zeta}}}
\newcommand{\etabf}{\ensuremath{\boldsymbol{\eta}}}
\newcommand{\thetabf}{\ensuremath{\boldsymbol{\theta}}}
\newcommand{\iotabf}{\ensuremath{\boldsymbol{\iota}}}
\newcommand{\kappabf}{\ensuremath{\boldsymbol{\kappa}}}
\newcommand{\lambdabf}{\ensuremath{\boldsymbol{\lambda}}}
\newcommand{\mubf}{\ensuremath{\boldsymbol{\mu}}}
\newcommand{\nubf}{\ensuremath{\boldsymbol{\nu}}}
\newcommand{\xibf}{\ensuremath{\boldsymbol{\xi}}}
\newcommand{\pibf}{\ensuremath{\boldsymbol{\pi}}}
\newcommand{\rhobf}{\ensuremath{\boldsymbol{\rho}}}
\newcommand{\sigmabf}{\ensuremath{\boldsymbol{\sigma}}}
\newcommand{\taubf}{\ensuremath{\boldsymbol{\tau}}}
\newcommand{\upsilonbf}{\ensuremath{\boldsymbol{\upsilon}}}
\newcommand{\phibf}{\ensuremath{\boldsymbol{\phi}}}
\newcommand{\chibf}{\ensuremath{\boldsymbol{\chi}}}
\newcommand{\psibf}{\ensuremath{\boldsymbol{\psi}}}
\newcommand{\omegabf}{\ensuremath{\boldsymbol{\omega}}}

% Greek letter uppercase boldface
\newcommand{\Gammabf}{\ensuremath{\boldsymbol{\Gamma}}}
\newcommand{\Deltabf}{\ensuremath{\boldsymbol{\Delta}}}
\newcommand{\Thetabf}{\ensuremath{\boldsymbol{\Theta}}}
\newcommand{\Lambdabf}{\ensuremath{\boldsymbol{\Lambda}}}
\newcommand{\Xibf}{\ensuremath{\boldsymbol{\Xi}}}
\newcommand{\Pibf}{\ensuremath{\boldsymbol{\Pi}}}
\newcommand{\Sigmabf}{\ensuremath{\boldsymbol{\Sigma}}}
\newcommand{\Upsilonbf}{\ensuremath{\boldsymbol{\Upsilon}}}
\newcommand{\Phibf}{\ensuremath{\boldsymbol{\Phi}}}
\newcommand{\Psibf}{\ensuremath{\boldsymbol{\Psi}}}
\newcommand{\Omegabf}{\ensuremath{\boldsymbol{\Omega}}}

\begin{document}
\begin{equation*}
\alphabf
\betabf
\gammabf
\deltabf
\epsbf
\epsilonbf
\zetabf
\etabf
\thetabf
\iotabf
\kappabf
\lambdabf
\mubf
\nubf
\xibf
\pibf
\rhobf
\sigmabf
\taubf
\upsilonbf
\phibf
\chibf
\psibf
\omegabf
\end{equation*}

\begin{equation*}
\Gammabf
\Deltabf
\Thetabf
\Lambdabf
\Xibf
\Pibf
\Sigmabf
\Upsilonbf
\Phibf
\Psibf
\Omegabf
\end{equation*}
\end{document}

\documentclass[12pt]{article}

\usepackage[papersize={35mm, 45mm}, text={30mm, 40mm}]{geometry}
\usepackage{concrete}
\usepackage{amsmath}

\DeclareMathOperator*{\AAbf}{\ensuremath{\mathbf{A}}}
\DeclareMathOperator*{\xxbf}{\ensuremath{\mathbf{x}}}
\DeclareMathOperator*{\bbbf}{\ensuremath{\mathbf{b}}}

\begin{document}
\begin{equation*}
\AAbf_{\scriptscriptstyle m\times n} \medspace
\xxbf_{\scriptscriptstyle n\times 1} =
\bbbf_{\scriptscriptstyle m\times 1}
\end{equation*}

\begin{equation*}
\AAbf_{m\times n} \medspace
\xxbf_{n\times 1} =
\bbbf_{m\times 1}
\end{equation*}

\end{document}