How tall you are and what is your chest measurement in inches (all around chest). I'm asking because most of diesel jackets and blazers run small.
Most diesel jackets run like this:
M - 40" chest
L - 42" chest
XL - 44" chest
(Not to confuse this with centimeters:)
In my case I'm 6'1" and my chest is 45 inches and diesel jackets size XL fits me quite tight.