How to Produce an Adjustable-Size Opening or Closing Curly Brace for Plain Text Only

The braces work in math mode, generally conforming to the size of the content by way of \left\{<content>\right. for only an opening brace (or \left.<content>\right\} for only a closing brace).

The slight "break" in the brace of the OP's image is an on-screen artifact associated with the magnification level (it's not really there).

With this approach, stackengine defaults to stacking text. That can be changed with a declaration \stackMath, which is not issued here. Thus, even in math mode, the "stack" defaults to text.

The \Centerstack is a vertically centered "Long" stack, that is, with each row spaced according to a fixed baselineskip (default \normalbaselineskip).

The default stack has center alignment, as shown. An optional argument to \Centerstack of [l] or [r] can change that to left or right.

The [usestackEOL] package option makes the default line break inside the stack to be \\, instead of a simple space .

\documentclass{article}
\usepackage[usestackEOL]{stackengine}
\begin{document}
$\left\{
\Centerstack{
the first sentence\\
the second sentence\\
the third sentence\\
the fourth sentence}
\right.$
\end{document}

All you have to do is set the interior structure using a tabular; this will set the contents in text mode rather than math mode (as it would with an array or similar environment):

\documentclass{article}

\begin{document}

$\left\{\begin{tabular}{@{} c @{}}
  the first sentence \\
  the second sentence \\ 
  the third sentence \\
  the fourth sentence
\end{tabular}\right.$

\end{document}