How can I make it that the font size of the label changes when I zoom in and out? I have a polygon and I want the label get bigger when I zoom in and smaller when I zoom out (just like the polygon does). I tried to use map points but it didn't work.
Try changing the size unit to Map Units. You might have to make this a very high number depending on the scale of your map, for instance in the project I'm working on at the moment at 1:260000 I have to set fonts to 700 map units to match the original 8pt text.