It is absolutely recommended to try the bra on in the shop to be sure you find your right fit as these sets are made in small batches with variance within each size, but here's a reference point as a guide :) A Midline Version of this bra is also available and it pairs well with the Bonnie Thong or Brief.
XS - 65A/B
S - 70B/C
M - 75B/C
L - 80C/D
XL - 85D
First photo is of @gab_kop by @tbehindthelens