My buddy is part of WHRW and gave me a tour of the place last summer. Wonderful place, particularly the archives. It's what got me into working at my own college radio station's archives.
That library is one of the largest in the Northeast. You probably saw Studio A, with the desk of boom mics? I paid for the microphone and preamp replacement back in... 2007, I think? It was a fun way to spend Christmas.
I couldn't tell you, we weren't there very long. I know the one studio I saw was off to the left when you walked in, and the library was more or less straight ahead. It didn't seem that large to me; or at least it's about the same size as my college's radio library. Maybe they're both considered large collections. My friend is in charge of the budget this year and is always rattling on about stuff I don't understand, mostly involving funding for various pieces of equipment. All I can say is he seems to enjoy it a lot.
We screwed up with the library's physical size when they moved the station from the 2nd floor of the old union to the basement of the new union. We needed more space as it was (we had about 35,000 recordings when I graduated in 1996, split between three rooms) and we wound up with about as much space as we already had. I would guess that there are 45-50k albums and CDs in that library, many of them impossible to replace.