Increase row height in align* cases environment

mathtools extends amsmath with dcases:

\documentclass{article}
\usepackage{mathtools}
\begin{document}

\[
A = \begin{dcases}
\frac{B-.5b}{C-.5c} & \text{sometimes} \\
\frac{D-.3d}{E-.7e} & \text{other times}
\end{dcases}
\]

\end{document}

The following shows how you can replicate cases using an array:

\documentclass{article}
\usepackage{amsmath}
\begin{document}

\begin{align*}
  A = \begin{cases}
    \frac{B-.5b}{C-.5c} & \text{sometimes} \\
    \frac{D-.3d}{E-.7e} & \text{other times}
  \end{cases}
\end{align*}
\begin{align*}
  A &= \begin{cases}
    \frac{B-.5b}{C-.5c} & \text{sometimes} \\
    \frac{D-.3d}{E-.7e} & \text{other times}
  \end{cases} \\
  A &= \renewcommand{\arraystretch}{1.5}\left\{\begin{array}{@{}l@{\quad}l@{}}
    \frac{B-.5b}{C-.5c} & \text{sometimes} \\
    \frac{D-.3d}{E-.7e} & \text{other times}
  \end{array}\right.\kern-\nulldelimiterspace
\end{align*}

\end{document}

The first align* replicates your output, while the second align* includes the original cases plus an array implementation. When using an array, you can adjust \arraystretch to stretch out the "cases" construction vertically (similar to other suggestions in Column and row padding in tables).

Note that the default \arraystretch for cases under amsmath is 1.2, as depicted in the \env@cases definition (taken from amsmath.dtx`):

\def\env@cases{%
  \let\@ifnextchar\new@ifnextchar
  \left\lbrace
  \def\arraystretch{1.2}%
  \array{@{}l@{\quad}l@{}}%
}

Of course you could change this default 1.2 stretch factor to something larger, but my assumption is that you want only a specific instance of cases to be slightly aired out rather than make a global change.

---

The size of the characters can be adjusted by using \dfrac instead of \frac. However, this would require a larger \arraystretch than 1.5. I'm not sure what is to be gained by this visually.

All that I did was to invoke \displaystyle for each line and to insert an extra blank line. EDITED (upon Werner's reminder) to use \dfrac instead of \displaystyle\frac, when the amsmath package is loaded.

\documentclass{article}
\usepackage{amsmath}
\begin{document}

\begin{align*}
A = \begin{cases}
\dfrac{B-.5b}{C-.5c} & \text{sometimes} \\
\\
\dfrac{D-.3d}{E-.7e} & \text{other times}
\end{cases}
\end{align*}

\end{document}