\documentclass{article}
\usepackage{xparse,xcolor}
\usepackage{ebgaramond} % fully scalable font
\usepackage{verbatim}

% egreg @ https://tex.stackexchange.com/questions/338364/
% egreg @ https://tex.stackexchange.com/questions/514372/
\ExplSyntaxOn

\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,
  color .initial:n = red,
  size  .initial:n = 1,
  skip  .initial:n = 1,
 }

\cs_new_eq:NN \optical_fontsize:nn \fontsize
\cs_generate_variant:Nn \optical_fontsize:nn { ee }


\NewDocumentCommand{\setmyfontsize}{ m O{1.2} }
 {
  \optical_fontsize:ee
   { \fp_eval:n { \use:c { f@size } + #1 } }
   { \fp_eval:n { ( \use:c { f@size } + #1 ) * ( #2 ) } }
  \selectfont
 }

\NewDocumentCommand{\stress}{ O{} m }
 {
  \group_begin:
  \keys_set:nn { mystyle } { #1 }
  \clist_map_inline:nn { #2 }
   {
    \mystyle:VVVn
     \l__mystyle_color_tl
     \l__mystyle_size_tl
     \l__mystyle_skip_tl
     {##1}
   }
  \group_end:
 }
\cs_new_protected:Nn \mystyle:nnnn
 {
  \setmyfontsize{#2}[#3]\textcolor{#1}{#4}
 }
\cs_generate_variant:Nn \mystyle:nnnn { VVV }
\ExplSyntaxOff

\begin{document}

\stress{emphasize this sentence with default values and add extra characters to check the default baselineskip}

\bigskip

\stress[color=green,size=-1]{\textit{emphasize this sentence with custom color and size values}}

\bigskip

\stress[color=red,size=1]{emphasize this sentence with default color and size values set explicitly and check the resulting baselineskip: should be the same baselineskip as on the first line}

\bigskip

\stress[color=blue,size=3]{emphasize this sentence with custom color and size values and check the default baselineskip for this size: should have bigger font and bigger baselineskip than above}

\bigskip

\stress[color=blue,size=3, skip=2]{emphasize this sentence with custom color size and baseline skip values and check the result: should have bigger font and much bigger baselineskip than above}

\bigskip

\stress[color=gray,size=3]{notice how a comma messes things, notice how a comma messes things}

\end{document}