I don't know the conversion. With mountain bikes, the frame is measured from the bottom bracket to the top tube. My mountain bike is a 16.5 inch frame, I ride a Kona.

Maybe the specialize web site can help you re: the sizing. If you have a friend that rides, bring them with you when you check out the bike. My gut says that 19.5 may be a little big--but without seeing you on the bike, I can't be sure.