In order to change the font size for a specific frame, you can issue any one of the font size changing commands at the beginning of the frame (similar to what is discussed in How do I use a particular font for a small section of text in my document?). For an interest in the specific size you're after, see What point (pt) font size are \Large etc.?
Below I've recreated your setup without changing the font size since the algorithm wouldn't fit otherwise. However, I've indicated where you can place your font size command. Other changes include redefining the way the control structures function - you seem to be after a grouping { ... } to delimit the scope.

\documentclass{beamer}
\usepackage{algorithm,algpseudocode}
\algrenewtext{For}[1]{\algorithmicfor\ (#1)\ \{}
\algrenewtext{EndFor}{\}}
\algrenewtext{If}[1]{\algorithmicif\ (#1)\ \algorithmicthen\ \{}
\algrenewtext{EndIf}{\}}
\algrenewtext{While}[1]{\algorithmicwhile\ (#1)\ \algorithmicdo\ \{}
\algrenewtext{EndWhile}{\}}
\algnewcommand{\Worklist}{\item[\textbf{Worklist:}]}
\begin{document}
\begin{frame}{Serial Breadth-First Search}
% \tiny \scriptsize \footnotesize \small % <------ make font smaller
% \normalsize % <--------------------------------- normal font size
% \large \Large \LARGE \Huge % <------------------ make font larger
\begin{algorithmic}[1]
\Require Graph G, Node root
\Worklist $wl$ = \{ root \};
\State root.level = 0;
\While{$wl \neq \emptyset$}
\State $u = DEQUEUE(wl)$;
\For{$v \in Vertices(G)$ such that $(u, v) \in Edges(G)$}
\hspace{.3cm} \textit{// Neighbors of u}
\If{v.level $>$ u.level + 1}
\State v.level = u.level + 1;
\State $ENQUEUE(wl, v)$;
\EndIf
\EndFor
\EndWhile
\end{algorithmic}
\end{frame}
\end{document}