What size?

#1

For the first time I am planning on making a garment for myself that isn’t socks or a blanket but I am not sure how to determine the correct size. I am going to be making the Kertzer Weekend Jacket SRK 180.

http://www.kertzer.com/Designs/DesignsCatalog/tabid/55/CategoryID/32/List/0/Level/a/ProductID/187/Default.aspx?SortField=DateCreated+DESC%2CProductNumber+DESC

For sizing it just gives finished measurements for bust (closed) in inches.

S. M. L. XL. XXL
37. 41. 45. 47. 53

How do I determine which one? :?? Do I go by my undergarment size or the measurement I get when measuring across my bust?:shrug:

Please help me. I am used to making sweaters for babies & children where they give sizes by age or actual kids clothing sizes.

#2

This looks like a pretty loose fitting jacket. I find that the best thing is to take a similar sweater or jacket that I like and measure across the width at the underarms (x2 for both the front and back). Otherwise, measure yourself across the bust and add 3-4" depending on how loose you want the jacket.