Rich text fields allow you to add stylings, such as font type and font size. Select the text to which you want to apply the styling before applying your formatting.
How Rich text field works with Word formatting:
Text will use styling specified in the Rich Text field (rich text is turned into HTML that gets converted by Word into XML). Font styling and font size will default to the Document Default styling.
IMPORTANT: when a Rich Text field is changed to Single or Multiline Text field, the Rich text value and formatting should be cleared first.