I was just researching this very thing moments ago. I wiped my OS for a Windows 8 install and didn't reinstall the Beta for the final weekend. Although I was all patched up through 1.11, I had been worried I was missing some kind of crucial data. Apparently, I'm not.
My Assets folder (c:\Program Files (x86)\Rift Game\Assets) is sitting at 12.2 gigs. Between all the Dimensions data that need be installed, and additional alterations to pre-existing files (basic patch overwrites) I should end up at the 15gb mark when all is said and done tomorrow.