Cannot remove font size for Textfield/StaticText from Design view->Font section. No reaction trying to remove font size manually or selecting empty value from drop-down.
Until the font size is presented directly in the component, font size is not applied from style/conditional style. To remove font size I need to find appropriate component in source view and delete size property directly from the source. F.e. for the font name it works properly - removing from design view removes the property from source code.