There is. The key is to configure the display and selection filter such that only your Reference Designators are chosen.
I assume from your question that you want to change ALL designations to the same size.
It depends a bit on how you treat your designators - we place them exclusively on the SS layers, others place them on the mounted layer (top or bottom) regardless, make that layer the current layer and turn off all others, also turn off unnecessary objects, (I usually just leave on REF DES and PADS). Set your filter to select only "Labels". Use CTRL-A or window select the whole board. RMB to bring up the properties dialog and enter the desired character height / line width.
Awesome! Just what I was looking for.