Set width column of a table with siunitx

Question

I would like to know how to set the column width even if I'm already defining the tabular with S. How can I define S and p at the same time?

\begin{table}[!h]
  \centering\renewcommand{\theadfont}{\small}
  \small\sisetup{per-mode=symbol}
  \setlength{\doublerulesep}{5pt}
  \rowcolors{3}{PalePink}{MyWhite}
  \begin{tabular}{|Cc||S[table-format=5.1]|S[table-format=3.1]|}
    \hhline{~--}
    \multicolumn{1}{c|}{} & {\thead{Molecular \\ weight \si{\g/\mol}}} & {\thead{Filterability \\ (\%)}} \\
    \hhline{-::==:}
    Water & 18 & 100 \\
    Sodium & 23 & 100 \\
    Chloride & 35.5 & 100 \\
    Potassium & 39 & 100 \\
    Urea & 60 & 100 \\
    Glucose & 180 & 100 \\
    Inulin & 5500 & 98 \\
    Myoglobin & 17000 & 75 \\
    Albumin & 69000 & 0.5 \\
    \hhline{-||--}
  \end{tabular}
    \caption{Molecular weight and filterability index of substances \cite{RefWorks:141}.}
  \label{ft_tab_ex}
\end{table}

Answer 1

You can set the width with table-column-width. As you did not provide a compilable MWE I had to guess the packages which you might use and remove a few things, such as the C columns, the colours etc.

\documentclass{article}

\usepackage{siunitx}
\usepackage[table]{xcolor}
\usepackage{makecell}
\usepackage{hhline}

\begin{document}


\begin{table}[!h]
  \centering\renewcommand{\theadfont}{\small}
  \small\sisetup{per-mode=symbol}
  \setlength{\doublerulesep}{5pt}
  \rowcolors{3}{red}{white}
  \begin{tabular}{|c||S[table-format=5.1,table-column-width=4cm]|S[table-format=3.1,table-column-width=4cm]|}
    \hhline{~--}
    & {\thead{Molecular \\ weight \si{\g/\mol}}} & {\thead{Filterability \\ (\%)}} \\
    \hhline{-::==:}
    Water & 18 & 100 \\
    Sodium & 23 & 100 \\
    Chloride & 35.5 & 100 \\
    Potassium & 39 & 100 \\
    Urea & 60 & 100 \\
    Glucose & 180 & 100 \\
    Inulin & 5500 & 98 \\
    Myoglobin & 17000 & 75 \\
    Albumin & 69000 & 0.5 \\
    \hhline{-||--}
  \end{tabular}
    \caption{Molecular weight and filterability index of substances \cite{RefWorks:141}.}
  \label{ft_tab_ex}
\end{table}


\end{document}

Answer 2

combination of S and p{...} column types. latter is used for column headers:

\documentclass{article}

\usepackage{siunitx}
\usepackage{ragged2e}
\usepackage[table]{xcolor}
\usepackage{array}
\newcommand\mcc[2]{\multicolumn{1}{>{\Centering}p{#1}|}{#2}}
\usepackage{hhline}

\begin{document}
    \begin{table}[!h]
    \centering
\sisetup{per-mode=symbol}
\renewcommand\arraystretch{1.2}
\setlength{\doublerulesep}{5pt}
\rowcolors{3}{red!30}{cyan!5}
    \begin{tabular}{|l||S[table-format=5.1]
                        |S[table-format=3.1]|}
    \hhline{~--}
\multicolumn{1}{c|}{}
            & \mcc{22mm}{Molecular weight \si{\g/\mol}} % <-- width of column determine `mcc`
                    & \mcc{22mm}{Filterability (\%)}    \\
    \hhline{-::==:}
Water       & 18    & 100                               \\
Sodium      & 23    & 100                               \\
Chloride    & 35.5  & 100                               \\
Potassium   & 39    & 100                               \\
Urea        & 60    & 100                               \\
Glucose     & 180   & 100                               \\
Inulin      & 5500  & 98                                \\
Myoglobin   & 17000 & 75                                \\
Albumin     & 69000 & 0.5                               \\
    \hhline{-||--}
    \end{tabular}
\caption{Molecular weight and filterability index of substances \cite{RefWorks:141}.}
\label{ft_tab_ex}
    \end{table}
\end{document}

(base for above answer is stolen from nice samcarter answer).