I'm not sure if there's a way to set this via the Settings menu, but you can edit system/config/adobeviewer.cfg and change the default fontsize in there.
If you're using Windows, then use an editor that allows you to switch to Unix-style line termination (i.e. ending each line with a linefeed rather than carriage return plus linefeed). I suspect that including carriage returns may break the configuration file.
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