The size of the carrier has nothing to do with your shirt size but everything to do with the size of your armor plate. When picking out a size, you will want to pick it to fit the hard armor plates you have.
The carrier itself will be adjustable, so that if your armor is sized correctly to cover your vitals then the correctly sized carrier will adjust to fit you properly.
There are two considerations when talking about plates, plate shape and plate size. This is complicated by manufacturers that will mix the two together, round their measurements, or come up with their own sizing entirely. There are two primary shapes or cuts, SAPI and Swimmer's
|SAPI/ESAPI Shape||8.75 x 11.75"||9.5 x 11.75"||10.125 x 13.25"||11 x 14"|
|Swimmer / Shooter||9.11 x 11.1"||9.92 x 11.6"||9.92 x 12.1||10.6 x 12.6"|
10 x 12" size plates are increasingly popular as it is a middle of the road size that covers many individuals. 10 x 12" is closest to a Medium SAPI or Large Swimmer cut, so most vests designed to these sizes will fit a 10x12" plate.