I sell in various venues at various times and I understand how difficult it can be to buy online when sizes are vague or not standard. I do my best to represent garments as accurately as possible.
My first suggestion is to get something out of your closet that has similar attributes. For example, a sweater with set in sleeves for a sweater with set in sleeves. You don't have to know exactly what set in sleeves are, just look for similarities. Don't measure something of yours that is sleeveless if you are looking at a sleeved garment.
You will need a tape measure.
Dritz Tape Measure- 5/8''x60''
First I lay the garment flat and neat on the table.
Measuring underarm to underarm.
Measuring length. Starts where the neck edge meets the collar.
Ends at the hem (this style has an open front, so the length is approximate))
From where the top of the sleeve meets the shoulder,
to the hem.