# Change fontsize of an array within an align environment

I am trying to label several lines at once in a very long equation. From this question I have come up with the following minimal example:

\documentclass{report}
\usepackage[onehalfspacing]{setspace}
\usepackage{amsmath}
\begin{document}
\begin{align*}
x &= \sum_i c_i X_i \\
&\left. \def\arraystretch{1.5}
\begin{array}{l}
+ \sum_i c_i Y_i \\
+\sum_i c_i Z_i
\end{array}
\right\} \tag{From (1)} \\
\end{align*}
\end{document}


which generates

As you can see I have attempted to correct the fact that array by default will have smaller row spacing than the align*. However, I cannot make the font sizes match up and the line spacing is really not that good either. I have tried using begingroup, endgroup but that doesn't seem to work. I would really appreciate any help on this!

If I have understood your problem correctly, you can make use of mathtools' drcases environment (d stands for displaystyle, while r means the brace will be on the right):

\documentclass[preview]{standalone}
\usepackage{mathtools}
\begin{document}
\begin{align*}
x &= \sum_i c_i X_i \\
&\hphantom{={}}\!\begin{drcases} % <--- \! is to get correct spacing/alignment
+ \sum_i c_i Y_i \\
+ \sum_i c_i Z_i
\end{drcases}
\tag{From (1)} \\
\end{align*}
\end{document}


If you want more space between the math and the brace, you can add \quad (or any other space) to the end of the longest line:

\documentclass[preview]{standalone}
\usepackage{mathtools}
\begin{document}
\begin{align*}
x &= \sum_i c_i X_i \\
&\hphantom{={}}\!\begin{drcases} % <--- \! is to get correct spacing/alignment

Another possibility (less "automatic") is to give the extra linespacing in square brackets after the \\ as in \\[.5em], which will give an extra 0.5 em of linespacing after the line you are ending.