Finished sizes from the book:

Extra Small (31.5”)
Small (35.5”)
Medium (39”)
Large (43”)
Extra Large (46.5”)

Based on the schematic, these finished sizes are prior to the buttonband which would add about another inch.

My intention was that the sizes should be Small through 2X when I wrote the pattern. But, if you prefer more ease in your garments, you will want to pick the sizes as written in the book.

As a point of reference, it looks to me that this model is wearing the garment with perhaps 1” ease. If you intend to wear the vest over a bulky sweater, you would want more ease.