what size should I buy if I weigh 165 LBS 5'4" tall and Waist 33" Small or Medium?
Mark, I should have said the SMALL is the correct size for you. The BC is fully adjustable so the Medium will fit as well - the Large would only work if you wear a heavily padded Dry Suit (meaning that you had lots of bulk on.) Sorry for the confusion.
I have the XL, I'm 6', 250lbs, and a 42" waist. I absolutely love this BC. BTW, I've over 2200 dives over 33 years so I've had my share of BC's. The Medium is the 'correct' choice for you. However, if you wear a drysuit, are a photographer, do double-tanks, or any other type of diving that requires a more demanding setup, you'd probably appreciate the 50lbs of lift the Large offers compared to the 35# of lift on the Medium. If you tend to be a warm-water "just looking" diver, then the Medium should be perfect. One thing, however, if you're a travel diver, this is not the right BC for a thrifty traveler. It is heavy (when wet, this BC with 18# weights, tank, and reg tips the scales at 75#!) and bulky (which is why I love it - it does everything and more with ease - you'll love the built-in goody bag which depoys out the bottom right pocket - I use it on many dives to put trash I pick up along the way), but I would definitely not travel with it. When I'm ready to travel, I'll buy a travel BC. I live in Hawaii so don't feel much need to travel ;-)
Medium. and you can grow into it.