In her book..."Robin Quivers, a Life" she recalled having sex with a guy who was hung like a donkey, but it did not appear to be too complimentary. I would think "normal" is her preference. I've been a Stern fan since the 80's (WNBC)...and never heard a preference.
I'm a huge fan of the show, and she was once asked about it. Her response to what size she prefers (I'm paraphrasing a bit), "Average. 7 inches is nice.". It was on the Wrap Up Show a few years ago when she had just become single.
I adore her, was glad she was doing the show from home last week, and Howard was sweet the day she was gone. Long time fan here, hope she is on the mend soon.