What are touch-friendly colors?
There isn't a built-in way to change font settings on the fly, other than editing them via the Preferences GUI. However, the config files are just XML text files so it would be possible to make a script or batch file which exits Opus, modifies the font sizes, then restarts Opus. (Or you could swap between two config files, but there are a lot of other settings in the file with the font sizes, so you probably would not want to do that.)
/dopusdata/Config Files/prefs.oxc is the file. If you do go down that path, be sure to keep backups in case any changes go wrong.