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

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.

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!