How do I edit PDF Annotation Text Properties after creation?
It does work in Lion, the way you tried it.  

In Snow-Leopard you have to select the text itself to change the font(size) by first clicking/selecting the text-field to get the selection rectangle (for re-sizing or moving - as you described) and then click a second time on your text to select all the characters. Now your text should be selected (visible by the highlighting) and you will be able to change the font-size in the Text Properties field.





Edit 2012-01-04:  

I could reproduce the problem described and also found a simple solution. If you open your pdf and re-arrange pages or rotate them and then try to change the formating of existing text fields it (sometimes) fails.  

The solution/fix is to click the window (menu-)bar of the "Fonts" window, then click on your text field again and finally chose the new font variant → and it will be applied :D