Let's try a small exercise on font sizes and see what happens.
Begin a new empty document; choose the size you want.
Draw a first frame in a page and type "All Is Well That Ends Well".
Set the font to Dustismo, if you have it (it is a GPL font that you can easily find, for example, at http://www.dafont.com/dustismo.font).
Duplicate the frame with Edit | Copy and Edit | Paste. The new frame is placed above the previous frame and you can move it down with the down arrow key. Keep both frames aligned anyways.
Set the font of this new frame to any other font (choose an Arial or Helvetica or any Sans standard font)
Just look at the difference.
When you change the font as you wish, some might have the same size exactly on screen and some others won't. In fact, you don't have many solutions to know the real font size: try and try and try again. In fact, 12pt or any measure doesn't tell you exactly how big the characters will be; it depends on the font...