How small should I go? I’m 19, 240 lbs, 38K. (Photos)

The
technique I recommend is The Bellesoma Method. 
This technique will reduce and reshape your breast tissue creating upper
pole fullness without implants, elevate them higher on the chest wall and more
medial to increase your cleavage.  The
weight of the breast is transferred to the underlying pectoralis major muscle
resulting in pain relief without excessive reduction, which allows you to
choose the size you desire.  Vertical
scars are avoided, nipple sensation and the ability to breast feed are maintained.Best Wishes,Gary Horndeski, M.D.

You seem to be a great candidate for a breast reduction, an in person consultation would be best for a detailed exam and evaluation.always see a board certified PS.

I would encourage you to consider getting another opinion if you were not comfortable with your first surgeons thoughts.  There are a lot of different techniques that can be done that preserve the nipple.  However, all large volume reductions have a chance of loss of sensation.  Many patients are still C to DD after reduction when they start with a large volume when an inferior pedicle technique is performed.  Also, some insurance companies may require more attempts at conservative measures such as physical therapy or weight loss before granting approval for surgery.   I usually recommend that patients are non smoking for 3 months and that they have their BMI at 35 or less to be the safest candidates for surgery with the lowest chances for complications such as loss of a nipple.  If you are not really convinced that now is the right time and it is not what you really want then I would consider waiting until you are really ready for surgery.  Best of luck on your journey.

You have breasts far out of proportion to your frame and need an breast reduction.   Quality of everyday life improves so much for breast reduction patients!   Perhaps get a second opinion prior to moving forward.  Your breasts should be in proportion to the rest of your body.   See a board certified plastic surgeon.  Good luck!

Thank you for the question and pictures. Based on your description, you seem to be an excellent candidate for breast reduction surgery. “Official” recommendations of course would necessitate in-person consultation.
Breast reduction surgery is one of the most patient pleasing operations we perform. By removing “excess” breast tissue, adipose tissue, and breast skin  this operation reduces AND lifts the breasts to a higher position on the chest wall (in other words, the breasts are "automatically" lifted when a breast reduction is performed). By doing so, patients often find improvement in neck, back, and shoulder discomfort and find it easier to form their activities of daily living and exercise.
When the time is right,  seek consultation with well-trained/experienced board-certified plastic surgeons. Ask to see lots of examples of their work and communicate your goals clearly. Best NOT  to communicate your goals in terms of achieving a specific cup size; this type of communication can be a source of miscommunication. In my practice, I prefer the use of goal pictures during this communication phase;  many patients wish to remove enough breast tissue to improve/alleviate symptoms while maintaining upper/lower body proportionality.
I hope this helps. Best wishes.