Fontsize in custom class with XeLatex and fontspec

I'm working on a couple of latex templates that belong together (letter, article, report). I've achieved nearly all my objectives except for a single one, having the font size change using the \documentclass[.pt]{} command, whether I use 9pt or 12pt my document looks the same. How do I ensure that this works correctly for my custom class?

Just some sidenotes, I use XeLaTex to compile my documents while I install the fonts (DejaVu familiy) with the fontspec package and the \setromanfont{DejaVu Serif} command. The classes have the option to switch between the serif and the sans-serif font in the family.

Here is my minimal working example: Consisting of the letter.tex file.

% !TeX encoding = UTF-8
% !TEX TS-program = xelatex

\documentclass[12pt, british]{letter-Bram}

% Sender information
\signature{Sender Full Name}
\address{Sender\\ Adress line 1\\ Adress line 2\\ Country}

\begin{document}
\begin{letter}{Recipient\\ Organization\\ Adress line 1\\ Adress line 2\\ Country}
    \opening{Dear Recipient,}
    This letter is send from the sender to the recipient, and styled in a sans-serif font. The class has the following class settings:
    \begin{description}
        \item[sans] To turn the document and math into a sans serif font.
        \item[lanuage] You can specify the language like normal. For example \enquote*{dutch}, \enquote*{british}, \enquote*{german}, defaults to \enquote*{american}. I recommend specifying a language as this does set quotation marks, date notations, hyphenation and others.
    \end{description}

    Here is an example equation:
    \begin{equation}
        a x^2 + b x + c = 0
    \end{equation}
    which can be solved with the standard $abc$ formula.

            
    \closing{Kind Regards,}
\end{letter}
\end{document}

And here is the letter-Bram.cls class file.

\NeedsTeXFormat{LaTeX2e}
\ProvidesClass{letter-Bram}[2019/07/01 v1.1 letter-Bram]

%%%%%%%%%%%%%%%%%%%%%
% NEW CLASS OPTIONS %
%%%%%%%%%%%%%%%%%%%%%
\newif\if@sans
\DeclareOption{sans}{\@sanstrue}
\DeclareOption*{\PassOptionsToClass{\CurrentOption}{article}}

\ProcessOptions\relax
\LoadClass{letter}

%%%%%%%%%%%%%%%%%
% LOAD PACKAGES %
%%%%%%%%%%%%%%%%%
\usepackage{babel}
\RequirePackage[fleqn]{amsmath} % needs before math font selection
\RequirePackage{csquotes} % proper quotation marks

%%%%%%%%%%%%%%
% LOAD FONTS %
%%%%%%%%%%%%%%
\RequirePackage{fontspec}
\setromanfont{DejaVu Serif}    % open serif font.
\setsansfont{DejaVu Sans}      % open sans font.
\setmonofont{DejaVu Sans Mono} % open mono font.
\if@sans
    \renewcommand{\familydefault}{\sfdefault}
    \RequirePackage{arevmath} % math font that is equal in style DejaVu Sans, but better a, nu, omega, l, x ect
    \SetSymbolFont{symbols}{bold}{OMS}{zavm}{m}{n} % surpress warning of bold math sybols
\else
    \usepackage{unicode-math}
    \setmathfont{DejaVu Math TeX Gyre} % math font equal in style and size to DejaVu Serif
    \SetSymbolFont{symbols}{bold}{OMS}{zavm}{m}{n}
\fi

I'm fairly sure that I'm missing something very obvious, but can't find it.

Solution

Two problems:

your class is loading the letter class, but pass it's option to the article class. Make up your mind, which of the two classes you actually want
9pt is not a valid option, try with 10pt vs. 12pt

\NeedsTeXFormat{LaTeX2e}
\ProvidesClass{letter-Bram}[2019/07/01 v1.1 letter-Bram]

%%%%%%%%%%%%%%%%%%%%%
% NEW CLASS OPTIONS %
%%%%%%%%%%%%%%%%%%%%%
\newif\if@sans
\DeclareOption{sans}{\@sanstrue}
\DeclareOption*{\PassOptionsToClass{\CurrentOption}{letter}}

\ProcessOptions\relax
\LoadClass{letter}

%%%%%%%%%%%%%%%%%
% LOAD PACKAGES %
%%%%%%%%%%%%%%%%%
\usepackage{babel}
\RequirePackage[fleqn]{amsmath} % needs before math font selection
\RequirePackage{csquotes} % proper quotation marks

%%%%%%%%%%%%%%
% LOAD FONTS %
%%%%%%%%%%%%%%
\RequirePackage{fontspec}
\setromanfont{DejaVu Serif}    % open serif font.
\setsansfont{DejaVu Sans}      % open sans font.
\setmonofont{DejaVu Sans Mono} % open mono font.
\if@sans
    \renewcommand{\familydefault}{\sfdefault}
    \RequirePackage{arevmath} % math font that is equal in style DejaVu Sans, but better a, nu, omega, l, x ect
    \SetSymbolFont{symbols}{bold}{OMS}{zavm}{m}{n} % surpress warning of bold math sybols
\else
    \usepackage{unicode-math}
%    \setmathfont{DejaVu Math TeX Gyre} % math font equal in style and size to DejaVu Serif
    \SetSymbolFont{symbols}{bold}{OMS}{zavm}{m}{n}
\fi