The two aspects of tables I'm explaining today are the column and row separation values. For those that are looking for quick fixes, I'll give them to you right away:
Column Width: \tabcolsep
Usage: \renewcommand\tabcolsep{6pt}
Explanation: tabcolsep is HALF the space between two columns. Default value is 6pt
Row Height: \arraystretch
Usage: \renewcommand\arraystretch{1}
Explanation: arraystretch is a FACTOR representing the distance between two rows. Default is 1
The two variables are manipulated in different ways: tablecolsep is a unit, so it can be specified in a variety of ways (pt, ex, em, etc...) which arraystretch is a factor, so it has no units.
The code demonstrates how to manipulate the variables, using beamer slides.
\documentclass{beamer}
\begin{document}
\begin{frame}[fragile]
\begin{tabular}{|c|c|c|c|c|c|c|c|c|c|c|c|c|c|c|}\hline
1 & 2 &3 &4 &5 &6 &7 &8 &9 & a &b &c &d &e &f \\\hline
Wide Column & & & & & & & & & & & & & & \\\hline
\end{tabular}
\renewcommand\arraystretch{1.1}
\renewcommand\tabcolsep{3pt}
\begin{tabular}{|c|c|c|c|c|c|c|c|c|c|c|c|c|c|c|}\hline
1 & 2 &3 &4 &5 &6 &7 &8 &9 & a &b &c &d &e &f \\\hline
Wide Column & & & & & & & & & & & & & & \\\hline
\end{tabular}
\renewcommand\arraystretch{0}
\renewcommand\tabcolsep{0pt}
\begin{tabular}{|c|c|c|c|c|c|c|c|c|c|c|c|c|c|c|}\hline
1 & 2 &3 &4 &5 &6 &7 &8 &9 & a &b &c &d &e &f \\\hline
Wide Column & & & & & & & & & & & & & & \\\hline
\end{tabular}
\renewcommand\arraystretch{2}
\renewcommand\tabcolsep{6pt}
\begin{tabular}{|c|c|c|c|c|c|c|c|c|c|c|c|c|c|c|}\hline
1 & 2 &3 &4 &5 &6 &7 &8 &9 & a &b &c &d &e &f \\\hline
Wide Column & & & & & & & & & & & & & & \\\hline
\end{tabular}
\end{frame}
\end{document}
Which produces these slides
