the experience I (and some other people) have made (as a "Tutor" in our residences) with single sex female flats was that they cause the most trouble out of all... (very sweeping generalization, I know, and I'm not going to elaborate on it).
I guess, it really depends on what kind of person you are, yet, personally, I think 5-9 people per flat is still not that much. 4-5 people could get a bit boring, and if you don't get along well with one or two this obviously has a much greater impact than among 9 students. Personally, I think around 8 students per flat is an ideal number - at my uni the residences with 6 people per flat tended to be relatively anti-social and dead, whereas those with 14-16 people per flat can be intimidating to some. Obviously, this all depends on the specific mixture of people per flat, layout and size of shared facilities, etc. Also, don't forget that in a lot of cases your primary group of friends are not necessarily going to be your flatmates (although it is nice if this does happen).
As to the boyfriend question (again out of the experience I gained dealing with complaints): this should generally be fine, but be careful not to occupy the shared facilities for too long and rather take him into your room. A male in the kitchen (for a longer period of time) can, for instance, cause distress to some strict Muslim girls.