These are a fairly straight up and down style of overall, not overly fitted around the waist.

The bust area is open at the sides, so waist and hip measurements are the important ones to find your fit. The waist does not need to be tight as the straps will hold them up.  You can adjust the bodice/total length by tying the straps to different lengths. 

Lucy is a size 12 (apple) and wears the L.

Tina is a Size 10 and wears the M

Garment Measurements

Size S M L XL XXL
Waist 86cm 94cm 100cm 105cm 110cm
Hips 100cm 110cm 118cm 122cm 128cm
Rough equivalent size 8 10 12 14-16 16-18