\documentclass[12pt]{article}
\usepackage[cp1251]{inputenc}
\usepackage[english,russian]{babel}
\usepackage[T2A]{fontenc}
\textwidth=170mm
\textheight=257mm
\oddsidemargin=-2mm
\evensidemargin=-2mm

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%   Задаем команды \lmy (левая угловая скобка), 
%                  \rmy (правая угловая скобка),
%                  \negmy (логическое отрицание) (размер 12pt - normalsize). 
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\font \kostintwelve   = kostin at 12pt
\font \kostinnine     = kostin at 9pt
\font \kostinsix      = kostin at 6pt
\newfam \kostinfam
\edef\kostinfamhex{\ifcase\kostinfam 0\or 1\or 2\or 3\or 4\or 5\or 6\or 7\or
         8\or 9\or A\or B\or C\or D\or E\or F\fi}

%   Для 12pt 
\textfont \kostinfam \kostintwelve            %   12pt = 100% от 12pt
\scriptfont \kostinfam \kostinnine            %    9pt =  70% от 12pt
\scriptscriptfont \kostinfam \kostinsix       %    6pt =  50% от 12pt

\mathchardef \lmy "4\kostinfamhex 31
\mathchardef \rmy "5\kostinfamhex 32
\mathchardef \parallelogram "\kostinfamhex 33
\mathchardef \tetrahedron "\kostinfamhex 34
\mathchardef \parallelepiped "\kostinfamhex 35
\mathchardef \negmy "\kostinfamhex 36

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%   Задаем команды \lmyss (левая угловая скобка), 
%                  \rmyss (правая угловая скобка),
%                  \negmyss (логическое отрицание) (размер 10pt - footnotesize). 
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\font \kostinten      = kostin at 10pt
\font \kostinseven    = kostin at 7pt
\font \kostinfive     = kostin at 5pt
\newfam \kostinfam
\edef\kostinfamhex{\ifcase\kostinfam 0\or 1\or 2\or 3\or 4\or 5\or 6\or 7\or
         8\or 9\or A\or B\or C\or D\or E\or F\fi}

%   Для 10pt
    \textfont \kostinfam \kostinten           %   10pt = 100% от 10pt
    \scriptfont \kostinfam \kostinseven       %    7pt =  70% от 10pt
    \scriptscriptfont \kostinfam \kostinfive  %    5pt =  50% от 10pt

\mathchardef \lmyss "4\kostinfamhex 31
\mathchardef \rmyss "5\kostinfamhex 32
\mathchardef \parallelogramss "\kostinfamhex 33
\mathchardef \tetrahedronss "\kostinfamhex 34
\mathchardef \parallelepipedss "\kostinfamhex 35
\mathchardef \negmyss "\kostinfamhex 36
                              
\begin{document}

\thispagestyle{empty}

Основной текст.

Рассмотрим пирамиду $ABCD$, где $A = \lmy 1,\; 0,\; -1 \rmy$,
$B = \lmy 3,\; -2,\; 1 \rmy$, $C = \lmy 1,\; 4,\; 5 \rmy$,
$D = \lmy -3,\; 1,\; 7 \rmy$.

\vspace{5mm}

\footnotesize

Вспомогательный текст.

Рассмотрим пирамиду $ABCD$, где $A = \lmyss 1,\; 0,\; -1 \rmyss$,
$B = \lmyss 3,\; -2,\; 1 \rmyss$, $C = \lmyss 1,\; 4,\; 5 \rmyss$,
$D = \lmyss -3,\; 1,\; 7 \rmyss$.

\end{document}