It’s interesting to study peasant clothes that use square and rectangular pieces, often utilizing the narrow widths of fabric available at the time, to make garments to fit a non-square body that needs room to move. In this blouse, the underarm gusset provides the extra room and comfort. The first photo shows the sleeve, a rectangular piece, that is stitched to the finished yoke, forming attractive pleats instead of gathers, and the cuff portion is adjustable with a string tie that fits it to the arm.
The photo below shows the square shape of the gusset that is sewn into the side seams of the sleeves and body.
The body in this blouse is one large piece sewn into a tube with the seam on one side only. This means that the gusset on the other side has to be inset into a slit cut into the folded edge. In the photo below, you can see how this was done, and that the stitching forms a dart under the gusset.
It seems to me that the important measurement in starting to draft a peasant blouse for yourself would be the shoulder tip to shoulder tip measurement.
Since the shapes are all rectangular or square, you can readily adjust the width of the yoke to fit yourself. You need to figure out how much ease you want in the body of the blouse which would depend on how drapey the fabric is. The tops I made for my daughter had about 8 to 10 inches of ease using a soft cotton. It’s also important to have the armhole depth deep enough. When I used the Vogue peasant blouse pattern, even though there were gussets the depth was too small and the blouse was very uncomfortable.
I think the next Mexican blouse I make will be white with tucks and laces.