I copied some Lorum Ipsum html formated text into the Scrivener window from loremipsum.io and the font size was set to -1. This cause me to be unable to change my font size until I closed and reopened the application
Which version are you using? If it;s the v. 3 Beta, you should post it in th Beta Testing forum where the devs will see it. But it will always help if you include information about your version of Windows and the Scrivener version.
Mark