When considering sizing, be aware of the cut of shirts. Some shirts are more tailored than others for a close fit, some call it race fit. Others are cut squarer for a relaxed fit or to cover spare tyres! I cannot describe every shirt, so please take a close look at the pictures and decide for yourself.
Jackets - I know it seems an obvious point to make, but, jackets fit over shirts. So although cycling shirts are thin, you need to take account of them, unless you don't need to breathe. So make sure you check measurements carefully and measure a jacket you know fits.
Measurements: chest measurements are of the shirts taken from armpit seam to armpit seam. So if your chest measures for example 42", then a 42" shirt would be skin tight and you would be unlikely to think it wearable. You should add on a couple of inches at least, three or four if you want it comfortable, to your actual chest measurement to be sure it wilI fit. I would highly recommend measuring a shirt that you currently find comfortable, armpit to armpit and comparing it to our measurements.