Something like the following, maybe?

The code uses the table* environment since it spans both columns. (LaTeX will place such a float at the top of a page automatically.) The width of the tabularx environment is set to \textwidth, not 2\textwidth. Note the use of the X column type for the first column and of the derived C column type for the remaining ones. I also suggest loading the textcomp package so that you can write \textmu m instead of um.
\documentclass{IEEEtran}
\usepackage{tabularx,textcomp}
\newcolumntype{C}{>{\centering\arraybackslash}X} % centered version of "X" column type
\usepackage{lipsum} % for filler text
\begin{document}
\begin{table*}
\setlength\tabcolsep{3pt} % default value: 6pt
\begin{tabularx}{\textwidth}{|X|C|C|C|C|C|C|C|C|C|C|C|C|} \hline
AES&Delay (ps)&WNS&Req time (ps)&Frequency&Total Cap (pF)&Tot Power (mW)
&Switch P (mW)&Int P (mW)&Leak P (mW)&Buff&Av length (\textmu m)& cells\\
\hline
Original Case &379&+1&380&0.4&8.2&1.28&0.5789&0.5771&0.124&2940&2.2574&12972\\
\hline
\end{tabularx}
\end{table*}
\lipsum[1-20] % filler text
\end{document}
\smallwould make the table smaller but whether or not that makes it fit I can not guess. – David Carlisle Feb 05 '15 at 23:53\begin{table*}and\end{table*}to make a two-column table, as described in section X-D "Double Column Floats" of the IEEEtran HOWTO? – Mike Renfro Feb 06 '15 at 01:19