With the following LaTeX code, the braces look too thick when the array is large. Is it possible to make the brace thinner? By the way, the | delimiters look always good.
\[\left\{\begin{array}{l}
A+B=C\\J+K=L
\end{array}\right. \quad
\left\{\begin{array}{l}
A+B=C\\J+K=L\\O+P=Q\\R+S=T\\U+V=W\\X+Y=Z
\end{array}\right. \]
\[\left|\begin{array}{l}
A+B=C\\J+K=L
\end{array}\right|\quad
\left|\begin{array}{l}
A+B+C\\J+K=L\\O+P=Q\\R+S=T\\U+V=W\\X+Y=Z
\end{array}\right| \]
Because other fotns are not wanted, the brace symbol can also be drawn by TikZ. It even provides a decoration with a brace:
\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{decorations.pathreplacing}
\newcommand*{\thinbraceleft}[1]{%
\vcenter{\hbox{%
\begin{tikzpicture}[
decoration=brace,
inner sep=0pt,
]
\node (M) {$\displaystyle #1$};
\draw[decorate] (M.south west) -- (M.north west);
\end{tikzpicture}%
}}%
}
\begin{document}
\[\left\{\begin{array}{l}
A+B=C\\J+K=L
\end{array}\right. \quad
\left\{\begin{array}{l}
A+B=C\\J+K=L\\O+P=Q\\R+S=T\\U+V=W\\X+Y=Z
\end{array}\right. \]
\[
\left\{
\begin{array}{l}
A+B=C\\J+K=L
\end{array}
\right.
\quad
\thinbraceleft{
\begin{array}{l}
A+B=C\\J+K=L\\O+P=Q\\R+S=T\\U+V=W\\X+Y=Z
\end{array}
}
\]
\[\left|\begin{array}{l}
A+B=C\\J+K=L
\end{array}\right|\quad
\left|\begin{array}{l}
A+B+C\\J+K=L\\O+P=Q\\R+S=T\\U+V=W\\X+Y=Z
\end{array}\right| \]
\end{document}
You can use different fonts, e.g. these offered by the package mnsymbol. But beware: this package makes other changes in appearance.
\documentclass{article}
\usepackage{mnsymbol}
\begin{document}
\[\left\{\begin{array}{l}
A+B=C\\J+K=L
\end{array}\right. \quad
\left\{\begin{array}{l}
A+B=C\\J+K=L\\O+P=Q\\R+S=T\\U+V=W\\X+Y=Z
\end{array}\right. \]
\[\left|\begin{array}{l}
A+B=C\\J+K=L
\end{array}\right|\quad
\left|\begin{array}{l}
A+B+C\\J+K=L\\O+P=Q\\R+S=T\\U+V=W\\X+Y=Z
\end{array}\right| \]
\end{document}
