Join Date: Oct 2003
Location: Asheville, NC
Programs: Virtuoso, FSPP, STARS, MO FAN Club, PEN Club, Bellini Club, Dorchester Diamond Club, InternovaSelect
Posts: 1,862
The usual parlance of hotel booking sites would suggest that Double refers to a one-bedded room ... and Twin refers to a two-bedded room. You should choose "Double." You'll get whatever is the bedding type for this room category - probably King.
If you then forward your reservation information to any of the FS preferred partners on FlyerTalk with the information regarding your booking, they'll be able to get your bedding preferences communicated to the hotel.