With each minor frustration with CS, I'm finding a solution which is either not documented or not covered very well.
Here's a tip for changing height of all reference designators on the PCB at one time. It's actually quite simple.
Make the PCB your active view.
Press F12 to bring up the PCB Filter (or type filter in the upper right search box, then the PCB filter shortcut)
Under PCB Filter, choose text under objects
ALL text is now highlighted
Here comes the magic part (sorry if it's been covered somewhere else but I couldn't find it).
You can click on a single reference designator ok, but a pick box will not pick them all up. You can hold shift down and continue selecting one by one, but a pick box with shift does nothing.
Hold CTRL down and use a pick box.
All designators are now chosen and will show up in the PCB Inspector panel.
Under Graphical, find Text Height. It will either show a single height if all the same, or will show <..> if they vary.
Type in your new height and hit enter.
Voila ! They all change at once.
If you have other text you do not want to change, you can either move it out of the way temporarily or make 2 or 3 passes at picking the designators, avoiding your other text.
Hope you find this useful !
added - I discovered that e14softwareuk had already posted a method of doing this. The method I've posted here seems (to me) a bit easier as the inspector panel doesn't always cooperate with me. Also, the e14softwareuk method chooses all items at once (CTRL-A) while here you can decide which to include via CTRL+pick box. YMMV
