How can I specify a font size for text that will take the same proportion of space regardless of screen resolution? For example I'd like the word "hello" to take half the screen's width, on an 800x600 screen and on a 1200x800 screen.

Point size doesn't seem to change the text's size when the width is smaller, is there something else I must do for point size to behave like that?

Thank you very much.