I use a sizing system of: XS, S, M, L, XL, XXL based on the European standard sizing. All pieces are true to size. Contact me with the designs you want and measure yourself around your waist, hips, widest part over your chest and under in cm. For better knowledge about your shapes include a photo of you in a bikini or unpadded underwear, it helps seeing your shapes how the bikini should be cut.
Between two sizes? Your right style and size will also depend on how you want to wear them – showing more skin, or less; sunbathing or surfing. I would recommend going for a size smaller if you are between two sizes since the fabric is super elastic and forms after your body. 
Most designs is designed with adjustable options to really get the perfect fit.