@SKT I usually wear size L in the IH workshirts. When I tried this on in L, it was a little snug in all the important spots for me.
This experience is of course only for this particular L, others may certainly run larger. I then went with an XL.
The best thing is to contact the crew of Iron Heart directly, they can certainly help you and find the right shirt for you.
But it is worth it in any case.
Cheers
Peter