You could use the tabularx environment instead of the tabular environment. The tabularx environment, made available by loading the tabularx package, provides a column type called "X" that should satisfy your professed need to have several equal-width columns. The first table in the MWE below explains how to do this.
There is one decision, however, that the author must make, i.e., which can't be automated entirely: How wide should the entire table be? Often, one simply sets this width to \textwidth (the width of the current textblock) and then lets tabularx determine how wide the individual columns of type X will be. For this MWE (which uses the default text font employed by LaTeX), with some trial-and-error work I found that it's possible to specify the table's overall width as narrow as about 0.75\textwidth. For your actual document, you will need to play around with the table width settings to determine empirically what the narrowest possible table width may be.
Note also the use of the specifier \centering\arraybackslash before the X column type in the MWE. I've inserted this specifier as it appears that you want the contents of the columns to be centered. (The contents of an X column are set "fully justified" by default.)
That said, I'd also like to very strongly encourage you to consider (i) getting rid of all vertical lines in the table and (ii) using the booktabs package to getter spacing above and below the various rules in the table. This package provides the commands \toprule, \bottomrule, \midrule, and \cmidrule; how to use them is explained in the second part of the MWE below. Note that it's possible to left-trim and/or right-trim a \cmidrule -- something that's quite impossible to achieve with \cline. Comparing the two forms of the table, I daresay predict that the overwhelming majority of readers will find the second form to be both more pleasing to look at and more easily understandable. :-)
\documentclass{article}
\usepackage{tabularx,booktabs}
\newcolumntype{Y}{>{\centering\arraybackslash}X}
\begin{document}
\noindent
\begin{minipage}{0.75\linewidth}
First version: Many vertical lines,
widths of horizontal lines all equal,
spacing above\slash below horizontal
lines cramped
\smallskip
\begin{tabularx}{\textwidth}{ |c| *{6}{Y|} }
\cline{2-7}
\multicolumn{1}{c|}{}
& \multicolumn{3}{c|}{Fantastical aardvarks}
& \multicolumn{3}{c|}{Spelunking elephants}\\
\hline
Foo bar & A & B & C & A & B & C\\
\hline
5 & 87 & 5 & 2 & 82 & 18 & 48\\
6 & 5 & 43 & 4 & 7 & 47 & 4\\
7 & 7 & 18 & 63 & 2 & 9 & 99\\
\hline
\end{tabularx}
\bigskip
Second version: No vertical lines,
widths of horizontal lines differ,
spacing above/below horizontal lines
satisfactory
\smallskip
\begin{tabularx}{\textwidth}{c *{6}{Y}}
\toprule
Foo bar
& \multicolumn{3}{c}{Fantastical aardvarks}
& \multicolumn{3}{c}{Spelunking elephants}\\
\cmidrule(lr){2-4} \cmidrule(l){5-7}
& A & B & C & A & B & C\\
\midrule
5 & 87 & 5 & 2 & 82 & 18 & 48\\
6 & 5 & 43 & 4 & 7 & 47 & 4\\
7 & 7 & 18 & 63 & 2 & 9 & 99\\
\bottomrule
\end{tabularx}
\end{minipage}
\end{document}
\multicolumn{1}{c|}{} & \multicolumn{3}{c|}{Fantastical aardvarks} & \multicolumn{3}{c|}{Spelunking elephants}. @EliLansey: You can use something smaller than\textwidth(or\linewidth), like.5\linewidthfor a 50% width table. – Werner Jun 20 '12 at 19:54Xcolumn widths will have to be at least 1/3 of the max of the string lengths of "Fantastical aardvarks" and "Spelunking elephants". (However, in your actual table you will have different entries, right?!) The minimum table width will also depend on factors such as the font and font size in use. Unless you have lots and lots of tables of this sort, you're probably best off going the trial-and-error route, i.e., try out0.8\textwdith,0.75\textwidth, etc until the table doesn't look right. – Mico Jun 20 '12 at 19:59Fantastical aardvarks. My suggestion just shifted the positioning of the vertical rules. A variation usingbooktabsis a clean alternative, since the table layout already imposes some horizontal organization of data, and therefore doesn't require more vertical rules. Read thebooktabsdocumentation for more on table design and layouts. – Werner Jun 20 '12 at 20:11(lr)option trims the rule on both the left and right, while the(l)option trims the rule only on the left-hand side. – Mico Jun 20 '12 at 20:38\begin{tabularx}{\textwidth}{ |c| *{6}{Y|} }with\begin{tabularx}{\textwidth}{ |*{7}{Y|} }and (b)Foo barwith\smash{\begin{tabular}[t]{@{}c@{}}Foo\bar\end{tabular}}. – Mico Jul 30 '18 at 11:42longtable? – Subhajit Paul Mar 13 '20 at 16:54longtableandtongtable. (The packages can't be used together.) Or, post a new query. – Mico Mar 13 '20 at 18:06