On Windows, these preferences are stored in a file at:
%APPDATA%\RStudio\desktop.ini
where %APPDATA% is an environment variable typically expanding to the directory AppData\Roaming within your user home folder.
In the [General] section of that file, you can set an entry font.fixedWidth to the font you want to use. For example, this is what my desktop.ini looks like:
[General]
RBinDir=C:/R/R-3.4.3patched/bin/i386
view.zoomLevel=1
font.fixedWidth=Consolas
[mainwindow]
geometry=@ByteArray(\x1\xd9\xd0\xcb\0\x2\0\0\xff\xff\xff\xf8\xff\xff\xff\xf8\0\0\n\a\0\0\x5\x7f\0\0\0\x6\0\0\0:\0\0\x5\x3\0\0\x5\x91\0\0\0\0\x2\0\0\0\n\0)
I'd love to know what's causing the crash, though -- would you mind generating a diagnostics report, just in case it provides any useful information?