I just finished sewing the tankini top for my 13-year old daughter. I chose size "R" because her bust size is 1/2" smaller than that. Her other measurements were dead-on for size "R." I used basic swimwear/dancewear lycra from JoAnnes. When done, the top fit like a glove, which means it was snug across her chest, waist, and hips. My daughter said it was comfortable, though, and wasn't too tight anywhere so the sizing was correct. It actually fit snugly like a one-piece swimsuit should fit.
I'm pointing this out in the review since I had imagined the fit to be a little bit looser like the tankini tops that I buy, and like the picture portrays. Because the fit is so true to the body, I will make a new one for my daughter in perhaps 2 sizes larger soon.
To add a little more give in the top that I did make, I added side gussets in a contrasting color which gave it real sporty look. I also added bra cups (looked up a how-to online) using the padding inserts from one of her existing bras. I had her try on the suit top with her bra on, then marked the place where the bra cups should go. After making sure they were symmetrically placed, I sewed them to the underside of the lining - very easy.