Style file with option to change pagesizes

Question

I would like to have a latex style file nutsi.sty that includes the my-definition feature. I also want to set the pagesize (21cm x 21cm) and sets the entire document in bold typeface.

Managed to make a latex file that uses nutsi.sty

    \documentclass{book}
    \usepackage{nutsi}
\end{document}



Although this style file sets a page-size of 21cm x 21cm, how can I
introduce an option that lets the user select either 1) a 21cm x 21cm
pagesize; 2) an A4 pagesize; or a B5 pagesize.
This is the file nutsi.sty.
\usepackage{geometry}
\geometry{ paperheight=21cm, paperwidth=21cm, left=8mm, right=8mm,
  top=21mm, bottom=21mm}

\usepackage{fix-cm}
\makeatletter
\renewcommand\normalsize{%
   \@setfontsize\normalsize{13pt}{15pt}
   \abovedisplayskip 10\p@ \@plus2\p@ \@minus5\p@
   \abovedisplayshortskip \z@ \@plus3\p@
   \belowdisplayshortskip 6\p@ \@plus3\p@ \@minus3\p@
   \belowdisplayskip \abovedisplayskip
   \let\@listi\@listI}
\makeatother

\usepackage{bm}                % makes bold arguments
\usepackage[x11names]{xcolor}  % loads 317 named rgb colours

\usepackage{amsmath}
\usepackage{amsthm}
\usepackage{amssymb}

% use boldface for whole document
\boldmath
\renewcommand{\seriesdefault}{\bfdefault}

\newtheoremstyle{my-definition} % name of style
{8pt}       % measure of space above definition, e.g. {3pt}
{8pt}       % measure of space below definition, e.g. {3pt}
{\itshape}  % name of font for body of example
{}          % measure of space to indent, e.g. {3pt}
{\bfseries\itshape}           % name of font for heading
{\textcolor{DarkOrange3}{.}}  % punctuation between heading and body
{ }         % space after theorem heading
{\thmname{\textbf{\textit{#1}}}
 \textcolor{DarkOrange3}{\textbf{\thmnumber{#2}}}
 \thmnote{\textit{({#3})}} }

\theoremstyle{my-definition}
\newtheorem{definition}{\textcolor{Blue3}{Definition}}[section]

Answer 1

The following uses expkv-opt to provide package options with a key=value interface.

With this you have one option paper that takes one of the three possible values 21, a4, and b5.

Please note that in a package you should use \RequirePackage instead of \usepackage, and you don't need \makeatletter.

nutsi.sty

\ProvidesPackage{nutsi}[2022-04-25 custom style by Ephram]
\RequirePackage{expkv-opt,expkv-def}
\ekvdefinekeys{nutsi}
  {
    choice-enum paper = \nutsi@paper {21, a4, b5}
  }
\ekvoProcessLocalOptions{nutsi}
\RequirePackage{geometry}
\ifcase\nutsi@paper
  \geometry{paperheight=21cm, paperwidth=21cm}
\or
  \geometry{a4paper}
\or
  \geometry{b5paper}
\fi
\geometry{left=8mm, right=8mm, top=21mm, bottom=21mm}
\RequirePackage{fix-cm}
\renewcommand\normalsize{%
   @setfontsize\normalsize{13pt}{15pt}
   \abovedisplayskip 10\p@ @plus2\p@ @minus5\p@
   \abovedisplayshortskip \z@ @plus3\p@
   \belowdisplayshortskip 6\p@ @plus3\p@ @minus3\p@
   \belowdisplayskip \abovedisplayskip
   \let@listi@listI}
\RequirePackage{bm}                % makes bold arguments
\RequirePackage[x11names]{xcolor}  % loads 317 named rgb colours
\RequirePackage{amsmath}
\RequirePackage{amsthm}
\RequirePackage{amssymb}
% use boldface for whole document
\boldmath
\renewcommand{\seriesdefault}{\bfdefault}
\newtheoremstyle{my-definition} % name of style
{8pt}       % measure of space above definition, e.g. {3pt}
{8pt}       % measure of space below definition, e.g. {3pt}
{\itshape}  % name of font for body of example
{}          % measure of space to indent, e.g. {3pt}
{\bfseries\itshape}           % name of font for heading
{\textcolor{DarkOrange3}{.}}  % punctuation between heading and body
{ }         % space after theorem heading
{\thmname{\textbf{\textit{#1}}}
 \textcolor{DarkOrange3}{\textbf{\thmnumber{#2}}}
 \thmnote{\textit{({#3})}} }
\theoremstyle{my-definition}
\newtheorem{definition}{\textcolor{Blue3}{Definition}}[section]

the document

\documentclass{article}
\usepackage[paper=b5]{nutsi}
\usepackage{duckuments}
\begin{document}
\duckument
\end{document}