I am sure someone will give a good answer here, however my question is, how many bedrooms are there without this "bedroom"? If there are 3-4 bedrooms and this one is an extra, and you can make some use of it, be more concerned about the square footage of the house than the number of bedrooms. That is what an appraiser takes into account. The only time an appraiser will deduct for bedroom count is if it is a house, not a condo or co-op, with 2 bedrooms, because it is considered a lot less desirable. Bathroom count does make a difference, but not the number of bedrooms.
That aside, the amount I have heard is 80 sq feet, with a closet.