I feed my 1.5 year old Boa one med. rat per week and she is between 4-5 feet long and in great shape...I haven't weighed her in a while, so I will try to get this info. If she looks thin, then feed her more. All snakes are different and grow at different rates. It is hard for anyone else to say how much/often yours should be eating.
I've tried feeding her more often and she just won't take it any sooner than 2 weeks. I guess I'll try upping her to a medium, but I'm a little afraid that will be too big, since the small is following the "slightly wider than the widest part of the body" rule.