That's tough to say. Long story short, I wouldn't base the size of the blazer you get off of your shirt size. It is very simple, however, to get a measuring tape and measure your own body dimensions. There are tons of websites that show you exactly how to do this. Then you can take these measurements and ask a tailor what size might best fit you. Or better yet, have a tailor take the measurements for you. Keep in mind also that manufacturers vary widely on the measurements they use for their various sizes. For instance, one manufacturer making a jacket sized 40 Regular might be vastly different from another manufacturer who puts the same size on their garments.