There is no dumb question.
The files that start with a dot are hidden files. When writing it's path as ~/.gtkrc.mine means it's in your home. (hence the ~ and the / that is the root of your home directory).
You must open it with a text editor (kwrite in Kde, Gedit in Gnome, Leafpad in Lxde, mousepad in Xfce are the ones most often default installed).
Then you will see a line that talks about the font size, just change the number there. Don't forget to save the change. Close the text editor. Reload X : quit the X session so that it be reloaded and come back to the X session.