Here are 2 problems you might be experiencing and how to fix them:
If you highlight your text and then choose the default font or size from the drop down menu, it may not work. All you have to do is select a different font or size first and then select the font or size you want.
If your text is highlighted when you click the 'Save' button, the changes you made to the highlighted portion will not be saved. Make sure to click elsewhere within the Editor before clicking the 'Save' button.