They turned this off many months ago due to the problems with the resizing of text. I hope they turn it back on again soon. I think it was back in begining of Febuary or something like that.
Untill then, I made a macro for doing just that. It saves a LOT of time when you chance characters or just logged in.
1) Choose the spot on the Main hotbar
2) Name your macro what ever you would like.
3) Create the macro ( /uiwinscale .6 - or what ever size you would like)
4) Check Text only macro
5) And hit create
Now when you log in, all you have to do is change over to the second tab in the Main hotbar, mouse over the windows you would like to resize, and hit that corrosponding number.
Below is an example of mine on one of my characters.. including the hotbar.
Note for those that do not know: 1.5 is HUGE and .5 is tiny