The measures in the Size guide are taken while all the laces are tied but, the lace can be worn looser leaving a slightly larger gap, or tightened with the edges touching.
Measure the smallest part of your waist and the widest part of your chest (across the nipples) tightly. Please avoid using a bra while measuring yourself.
If your measurements are between sizes, I would recommend you to use the smaller size, in this way, the corset will fit fine and only the gap is going to be a little bit larger, otherwise the corset will be loose and will not hold you.
If you are worried or have a larger bust/waist than what is listed, I strongly suggest you to send me your measurements together with the fabrics of your choice in the order notes at check out and I'll adjust the pattern size accordingly.