For a BBL are there sizes you can choose from? Or is it all one specific size?

helllo thank you for your question and provided inforamtion as well...based on the case I see that you you can  breast lift with small implant, My recommendation is that you get a consultation in person with a certified plastic surgeon to see your case.

Without any picture provided it is impossible to tell if you are a candidate.
 A  Brazilian butt lift harvest fat from areas  with the fat removed from the areas using your own fat it transfer to the buttocks and into the hips.The common donor sites for this procedure is your posterior waist/flanks, lateral thighs, back, waist, and anterior tummy.
 Choose a board certified plastic surgeon who is highly experience in Brazilian Butt Lifts.  

ANSWER: The amount of fat grafted to the buttock for a Brazilian Butt Lift (BBL) is specific to each individual patient. Factors that affected how much fat is grafted is based on skin elasticity, as some patients have very tight skin or a very small frame and these are variables that would limit the amount that can be transferred versus a patient with loose skin or a wider frame. Also,  injecting too much fat in a very tight place may hinder the ability for the fat to gain a blood supply and survive. It is best that you consult with a board certified plastic surgeon who is experienced in performing BBL and discuss your desires to ensure you expectations are attainable. Hope this answers your question.Best of luck,Andrew Jimerson II, MD

Hello dear!!
Thanks for the question and provided information as well. 
It depends on the amount of fat for every patient. It is possible to grasp fat from your back, waist, flanks and lateral thighs.
l recommend you to make an appointment with a board certified plastic surgeon to talk about your goals and anything else you need to know.Good luck :)

The size depends on the amount of fat available for the transfer and the skin elasticity. Seek a plastic surgeon who performs many of these procedures, has great reviews and photos, and is an expert in this field.