Every body is different. You may have found that size guides can be unreliable; a size 12 in one shop can have completely different measurements and proportions to a size 12 in another shop. Each brand has their own size guide which is based on averages from mass surveys and the typical figure of their target demographic. However mass produced garments cut according to an average cannot possibly accommodate the complexity of each individual.
This Once Was garments are designed to be as versatile and adjustable as possible. The X-over trousers are suitable for all sizes. The measurements of each of the tops and jackets are given so that you can measure yourself and see if it will fit. Owning a tape measure and getting used to your measurements will help you better understand your shape so it is good to get acquainted with the process! Then there is no guess work and you can buy according to your body and not a constrictive, standardised size system.