\RequirePackage{fix-cm}
\documentclass{article}
\usepackage{xcolor}
\usepackage{verbatim}

\ExplSyntaxOn

\tl_new:N \l__mystyle_style_tl

\keys_define:nn { mystyle }
 {
  color .tl_set:N = \l__mystyle_color_tl,
  size  .tl_set:N = \l__mystyle_size_tl,
  skip  .tl_set:N = \l__mystyle_skip_tl,

  style .choice:,
  style/it .code:n = \tl_set:Nn \l__mystyle_style_tl { \itshape },
  style/bf .code:n = \tl_set:Nn \l__mystyle_style_tl { \bfseries },
  style/bfit .code:n = \tl_set:Nn \l__mystyle_style_tl { \bfseries\itshape },
  style/unknown .code:n = \tl_set:Nn \l__mystyle_style_tl { #1 },

  color .initial:n = red!75,
  size  .initial:n = 12pt,
  skip  .initial:n = 14.5pt,
 }
\NewDocumentCommand{\stress}{O{}m}
 {
  \group_begin:
  \keys_set:nn { mystyle } { #1 }
  \mystyle:VVVVn
    \l__mystyle_color_tl
    \l__mystyle_size_tl
    \l__mystyle_skip_tl
    \l__mystyle_style_tl
    {#2}
  \group_end:
 }
\cs_new_protected:Nn \mystyle:nnnnn
 {
  \leavevmode\color{#1}\fontsize{#2}{#3}#4\selectfont#5
 }
\cs_generate_variant:Nn \mystyle:nnnnn { VVVV }

\ExplSyntaxOff

\begin{document}

\stress{a very long sentence a very long sentence a very long sentence a very long sentence a very long sentence a very long sentence a very long sentence a very long sentence a very long sentence a very long sentence a very long sentence a very long sentence a very long sentence a very long sentence a very long sentence}

\stress[skip=50pt]{a very long sentence a very long sentence a very long sentence a very long sentence a very long sentence a very long sentence a very long sentence a very long sentence a very long sentence a very long sentence a very long sentence a very long sentence a very long sentence a very long sentence a very long sentence}

\stress{a sentence with a comma HERE, now works!}

\end{document}