Is your OS set to use a high DPI?
If so, it sounds like this issue:
[Font size changing bug)
To check the DPI setting in Windows 7, right-click the desktop and choose Screen Resolution, then click Make text and other items larger or smaller. If it's not set to the Smaller - 100% (default) option then it's in high DPI mode.
As far as I can tell, there is a bug in Windows where sometimes it thinks Opus is high-DPI aware and sometimes it thinks it isn't. (That definitely happens, but I'm not certain it's what causes the font-size issue for you and the poster of the other thread. It's just a theory so far. If your OS isn't set to use high DPI then the theory can't be true. On the other hand, if I'm right then it should be fixed by making Opus high DPI aware, which is planned for a future version.)