I am using the longtable environment in combination with the siunitx package for 3 of my 10 columns. Unfortunately, when compiling the spacing of those three columns is not automatically adjusted. When using table-column-width the spacing of my top row with comments to the section is not adjusted accordingly. Please see my code (excerpt) and the output below, first without using the table-column-width command and then with respective command.
\documentclass[11pt,a4paper,oneside]{article}
\usepackage{setspace}
\onehalfspacing
\usepackage[round]{natbib}
\usepackage{array}
\usepackage{pdflscape}
\usepackage{longtable}
\usepackage[group-separator={.}]{siunitx}
\usepackage{booktabs,multirow}
\renewcommand{\thetable}{\Roman{table}}
\usepackage[english]{babel}
\usepackage[margin=1in]{geometry}
\usepackage[small,bf,labelsep=period]{caption}
% -------------------------------------------------------------------
\begin{document}
\newpage
\begin{landscape}
{\small\begin{longtable}{lccccccS[table-format=2.4, table-column-width=60pt]S[table-format=1.4, table-column-width=60pt]S[table-format=1.4, table-column-width=60pt]}
\caption[Significant contributions]{Significant contributions} \label{tab:contributionscharacteristics} \\
\multicolumn{10}{p{.9\linewidth}}{Lorem ipsum dolor sit amet, consectetur adipiscing elit. In pharetra lobortis elit nec consectetur. Nulla accumsan lacus a diam varius, eget eleifend augue egestas. Nulla porttitor lacus turpis, in elementum tortor pellentesque sit amet. Mauris interdum ante ac velit fermentum, sed faucibus nisi elementum. Duis pretium lacinia nunc, quis maximus lectus facilisis id. Aliquam ornare dapibus risus, sed congue quam eleifend id. Donec vel fringilla lacus. Aenean blandit viverra quam, nec imperdiet ante volutpat ut. Mauris non dapibus eros. Suspendisse luctus feugiat tortor, quis consectetur metus facilisis vitae. Aenean imperdiet est sodales arcu ultrices lacinia. Duis nunc augue, egestas nec lacus ac, egestas porttitor nibh. Donec sed lorem sit amet ipsum vulputate consectetur in ut lorem. Nunc mattis luctus magna. } \\
\\
\hline
\rule{0pt}{3ex}
\multirow{2}{*} {{Author(s) and country of analysis}}&\multicolumn{2}{c} {{Sample}} & \multicolumn{2}{c} {{Event}} & \multirow{2}{*} {{Weighting}} & \multirow{2}{*} {{Type\textsuperscript{b}}} & \multicolumn{3}{c}{\hspace{15pt}{Key results}} \\
& \multicolumn{1}{c}{Period} & \multicolumn{1}{c}{Size} & \multicolumn{1}{c}{Window}\textsuperscript{a} & \multicolumn{1}{c}{Date}\textsuperscript{b} & & & \multicolumn{1}{c}{BHAR}& \multicolumn{1}{c}{Alphas}& \multicolumn{1}{c}{CTAR}\\
\hline \\ [-2ex]
\endfirsthead
\multicolumn{10}{c}%
{\small{{\bfseries \tablename\ \thetable.{}} Significant contributions (continued)}} \\ [1ex]
\hline
\endhead
\hline \multicolumn{10}{r}{{Continued on next page}} \\
\endfoot
\endlastfoot
\cite{Loughran1997}, US & 1970-1989 & 234 & $[+1, +60]$ & Co& EW & C & 0.3050\textsuperscript{1,2}* & & \\
& &300 & & & EW & S & -0.2400\textsuperscript{1,2}*** & & \\
\cite{Mitchell2000}, US & 1961-1993 & 1,029 & $[+1, +36]$ & Co & EW & S & -0.0840\textsuperscript{1}*** & -0.0900\textsuperscript{7}*** & -0.0828\textsuperscript{4}*** \\
& & & & & VW & S & -0.0530\textsuperscript{1}***& -0.0432\textsuperscript{7}*& -0.0468\textsuperscript{4}***\\
& & 1,039 & & & EW & NC & 0.0640\textsuperscript{1}**& -0.0144\textsuperscript{7}*& -0.0252\textsuperscript{4}\\
\hline
\end{longtable}
}
\end{landscape}
\end{document}
Without table-column-width command
With table-column-width command, as visible the section above with the description does not allign nicely