LO 5 - Can't change font size within form fields!

LO 5.0.3.2 on Win10:
I’m using form fields in a writer document for mail merge. I have set a font size within the properties of the fields and in the writer window it is Shown correctly. But when i print or use the print preview the font of the form fields is too small. When i zoom within the print preview the font within form fields keeps still the same!!!