The size thing is pretty hard to pin down, since as you mentioned sizing has changed quite a bit over time and varies dramatically by country, designer, manufacturer, etc.
According to measurements from Marilyn Monroe's dressmaker:
Height: 5 feet, 5½ inches
Weight: 118-140 pounds
Bust: 35-37 inches
Waist: 22-23 inches
Hips: 35-36 inches
Bra size: 36D
(published several places. I found it at:
http://jezebel.com/5299793/for-the-last-time-what-size-was-marilyn-monroe#ixzz0vBl0a4zJ)
Which would mean she would probably buy a modern ready-to-wear size 4-6 and would need to have it tailored significantly to nip in at the waist.