What should the weight be for someone that's 5'4"-5'5", in order for them to get a BBL?

Thanks for your question.
At my practice, the max Body Mass Index (BMI) limit for a Brazilian butt lift (BBL) is 35. There are BMI calculators online that can tell you what you would need to weigh for your height in order to be at or under a 35 BMI. 
I suggest you schedule a virtual consult with a board certified plastic surgeon for a proper evaluation. 
Stay safe and take care.

Moises Salama MD, board certified plastic surgeon

There is no ideal weight. This will depend upon where your fat is distributed and your goals for redistribution.  Overall medical health and suitability of the procedure are just as important as the BMI. In my practice I look for a BMI approximating 30 to lessen the risks of complications during and after surgery and to enhance the results of the procedures. A BMI of 35 is probably the upper limit that a plastic surgeon would consider for this procedure. It is best to be within 10 pounds of your weight goal for three months prior to the BBL.

Dear 0504tee,
Generally speaking, you want to reach your goal weight before you consider a BBL, otherwise your results will be suboptimal if you continue to lose weight after the surgery.
If you are considering a surgery, I would suggest you to consult a board-certified plastic surgeon. Only after a thorough examination you will get more information and recommendations.
Daniel Barrett, MD
Certified, American Board of Plastic Surgery
Member, American Society of Plastic Surgery
Member, American Society of Aesthetic Plastic Surgery

A healthy BMI is between 15 - 24, the maximum recommended being around 30. If you are above 30, then you are encouraged to lose weight before undergoing surgery. The higher the BMI the higher the chance of complications. A Skype or FaceTime consultation with a board certified plastic surgeon will help you get an accurate recommendation for your procedure and for pricing. 
Respectfully, 
Arian Mowlavi MD, FACS
Realself 100 Hall of Fame Inductee

The ideal weight for someone 5 foot 4 to 5 foot five for a BBL is 145 pounds.
Under 120 pounds it may be challenging the harvest sufficient amounts of fat to justify the procedure though patients have had excellent outcomes at that height with weights lower than 120 pounds. above 170 pounds and patients cannot have the maximum benefit of Liposuction with one operation if the sturgeon stays within the safety limits in regards to fat removal. 
To get a deal results with torso or body liposuction patients over 170 pounds after need more than 5 L of fat removed.
Weight or BMI is only one variable in regards to candidacy for the BBL.
There are many other variables that need to be assessed in particular skin laxity, skeletal structure, presence of cellulite, muscle separation from previous pregnancies, excess visceral fat etc. etc. etc.
I suggest scheduling several in person consultations for the local board-certified plastic surgeons in your area.

Best, 
Mats Hagstrom MD