How to make the \frametitle row smaller in berkeley theme in beamer

Set the height of the header in your preamble by adjusting \beamer@headheight accordingly:

\documentclass[14pt]{beamer}% http://ctan.org/pkg/beamer
\let\Tiny\tiny% http://tex.stackexchange.com/q/58087/5764
\usetheme{Berkeley}
\makeatletter
\beamer@headheight=1.5\baselineskip
\makeatother
\setbeamercolor{normal text}{bg=black!10}
\begin{document}
\title[Title]{My title}
\subtitle{Subtitle}
\author{Author}
\institute[Institute]{My institute}
\date[Date]{My date}
\logo{\color{blue!50}\scalebox{2}{\TeX}} % you can % it
\begin{frame}
  \titlepage
\end{frame}

\section{A section}
\subsection{A subsection}
\begin{frame}
  \frametitle{Frame title}
  \framesubtitle{frame subtitle}
  Some text
\end{frame}

\begin{frame}
  Some more text
\end{frame}

\section{Another section}
\subsection{Another subsection}
\begin{frame}
  Some final text
\end{frame}
\end{document}

The default is 2.5\baselineskip, which results in 62.5pt (2.5 x 25pt) under the 14pt base font you selected.