You can place the algorithmic
environment inside a minipage
of desired width (for example, 1.5\columnwidth
):

\documentclass{IEEEtran}
\usepackage{algpseudocode}% http://ctan.org/pkg/algorithmicx
\usepackage{lipsum}% http://ctan.org/pkg/lipsum
\usepackage{float}% http://ctan.org/pkg/float
\floatstyle{boxed} % Box...
\restylefloat{figure}% ...figure environment contents.
\begin{document}
\section{A section}
\begin{figure*}
\centering
\caption{Euclid’s algorithm}\label{euclid}
\begin{minipage}{1.5\columnwidth}
\begin{algorithmic}[1]
\Procedure{Euclid}{$a,b$}\Comment{The g.c.d. of a and b}
\State $r\gets a\bmod b$
\While{$r\not=0$}\Comment{We have the answer if r is 0}
\State $a\gets b$
\State $b\gets r$
\State $r\gets a\bmod b$
\EndWhile\label{euclidendwhile}
\State \textbf{return} $b$\Comment{The gcd is b}
\EndProcedure
\end{algorithmic}
\end{minipage}
\end{figure*}
\lipsum[1-15]% dummy text
\end{document}
To reduce the box size itself, it is far more convenient to remove the boxed
float style, and mere wrap the minipage
d algorithmic
environment inside an \fbox
:

\documentclass{IEEEtran}
\usepackage{algpseudocode}% http://ctan.org/pkg/algorithmicx
\usepackage{lipsum}% http://ctan.org/pkg/lipsum
\usepackage{float}% http://ctan.org/pkg/float
%\floatstyle{boxed} % Box...
\restylefloat{figure}% ...figure environment contents.
\begin{document}
\section{A section}
\begin{figure*}
\centering
\caption{Euclid’s algorithm}\label{euclid}
\fbox{\begin{minipage}{1.5\columnwidth}
\begin{algorithmic}[1]
\Procedure{Euclid}{$a,b$}\Comment{The g.c.d. of a and b}
\State $r\gets a\bmod b$
\While{$r\not=0$}\Comment{We have the answer if r is 0}
\State $a\gets b$
\State $b\gets r$
\State $r\gets a\bmod b$
\EndWhile\label{euclidendwhile}
\State \textbf{return} $b$\Comment{The gcd is b}
\EndProcedure
\end{algorithmic}
\end{minipage}}
\end{figure*}
\lipsum[1-15]% dummy text
\end{document}