@Zinc2162 Do you have a regular Iron Heart Shirt to compare to? In my experience, IH CPO shirts tend to be roughly one size larger than a regular IH shirt in the same tagged size, but that's not a hard and fast rule.
I find that the IHSH-23-OLV and the GRY and BLK tend to run a bit smaller and trim in the arms. I needed to upsize to an XXL in this shirt, but sometimes I can get away with an XL in some other Iron Heart shirts. I can typically wear an XL in a CPO if I'm using it like a shirt not a jacket.
In my opinion, if you're unsure, go up a size in the Serge. The XXL is perfect for me in the Military Serge. I think this is because the Serge material stretches less than denim and chambray fabrics, and can be less flexible in general. Does this make sense? I find other fabrics to be be more "forgiving"?