I don't know what 'the font size drop down' is, but this message appears is when you are about to leave the page or reload it and you are at risk of losing your edits on that page.

You can disable this protection in your user preferences in b2evolution.

I would say then that either the page is being re-written when certain editor selections are made
(I cant see a refresh) or the event is false triggered. either way the pop-up is not appropriate or desirable
at that point.

Font size drop down - one of the options on the post editing form.
Other selections on the form do the same.

Go to your user profile in the back office, advanced tab and uncheck the option "control form closing"